Replace bootwrapped kernel instructions from User Guide

The instructions to boot the bootwrapped kernel were outdated.

Also, the bootwrapped kernel boot flow isn't really useful. It was meant
to be a replacement for the Trusted Firmware-A, not to be used as the next
step during boot.

The instructions have been removed in favour of the new build option
ARM_LINUX_KERNEL_AS_BL33. This new system directly boots the Linux
kernel from BL31, and requires RESET_TO_BL31 to be 1. Also, the kernel
has to be preloaded in memory, so PRELOADED_BL33_BASE has to be set to its
address. This way, the runtime services of the Trusted Firmware-A are
available for the kernel in the least possible amount of time.

This new system requires the DTB to be patched so that the kernel knows
where the ramdisk is. A short script to add this information to the DTB
has been added to the User Guide. The information related to it can be
found in the following file in the Linux kernel tree:
``Documentation/devicetree/bindings/chosen.txt``

Change-Id: Ide135580959e09f6aa8e4425f37ea55d97439178
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

docs/user-guide.rst

@@ -1484,41 +1484,92 @@ without a BL33 and prepare to jump to a BL33 image loaded at address
 
     make PRELOADED_BL33_BASE=0x80000000 PLAT=fvp all fip
 
-Boot of a preloaded bootwrapped kernel image on Base FVP
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Boot of a preloaded kernel image on Base FVP
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-The following example uses the AArch64 boot wrapper. This simplifies normal
-world booting while also making use of TF-A features. It can be obtained from
-its repository with:
+The following example uses a simplified boot flow by directly jumping from the
+TF-A to the Linux kernel, which will use a ramdisk as filesystem. This can be
+useful if both the kernel and the device tree blob (DTB) are already present in
+memory (like in FVP).
+
+For example, if the kernel is loaded at ``0x80080000`` and the DTB is loaded at
+address ``0x82000000``, the firmware can be built like this:
 
 ::
 
-    git clone git://git.kernel.org/pub/scm/linux/kernel/git/mark/boot-wrapper-aarch64.git
+    CROSS_COMPILE=aarch64-linux-gnu-  \
+    make PLAT=fvp DEBUG=1             \
+    RESET_TO_BL31=1                   \
+    ARM_LINUX_KERNEL_AS_BL33=1        \
+    PRELOADED_BL33_BASE=0x80080000    \
+    ARM_PRELOADED_DTB_BASE=0x82000000 \
+    all fip
+
+Now, it is needed to modify the DTB so that the kernel knows the address of the
+ramdisk. The following script generates a patched DTB from the provided one,
+assuming that the ramdisk is loaded at address ``0x84000000``. Note that this
+script assumes that the user is using a ramdisk image prepared for U-Boot, like
+the ones provided by Linaro. If using a ramdisk without this header,the ``0x40``
+offset in ``INITRD_START`` has to be removed.
+
+.. code:: bash
+
+    #!/bin/bash
+
+    # Path to the input DTB
+    KERNEL_DTB=<path-to>/<fdt>
+    # Path to the output DTB
+    PATCHED_KERNEL_DTB=<path-to>/<patched-fdt>
+    # Base address of the ramdisk
+    INITRD_BASE=0x84000000
+    # Path to the ramdisk
+    INITRD=<path-to>/<ramdisk.img>
+
+    # Skip uboot header (64 bytes)
+    INITRD_START=$(printf "0x%x" $((${INITRD_BASE} + 0x40)) )
+    INITRD_SIZE=$(stat -Lc %s ${INITRD})
+    INITRD_END=$(printf "0x%x" $((${INITRD_BASE} + ${INITRD_SIZE})) )
 
-After compiling it, an ELF file is generated. It can be loaded with the
-following command:
+    CHOSEN_NODE=$(echo                                        \
+    "/ {                                                      \
+            chosen {                                          \
+                    linux,initrd-start = <${INITRD_START}>;   \
+                    linux,initrd-end = <${INITRD_END}>;       \
+            };                                                \
+    };")
+
+    echo $(dtc -O dts -I dtb ${KERNEL_DTB}) ${CHOSEN_NODE} |  \
+            dtc -O dtb -o ${PATCHED_KERNEL_DTB} -
+
+And the FVP binary can be run with the following command:
 
 ::
 
     <path-to>/FVP_Base_AEMv8A-AEMv8A                            \
-        -C bp.secureflashloader.fname=bl1.bin     \
-        -C bp.flashloader0.fname=fip.bin          \
-        -a cluster0.cpu0=<bootwrapped-kernel.elf> \
-        --start cluster0.cpu0=0x0
-
-The ``-a cluster0.cpu0=<bootwrapped-kernel.elf>`` option loads the ELF file. It
-also sets the PC register to the ELF entry point address, which is not the
-desired behaviour, so the ``--start cluster0.cpu0=0x0`` option forces the PC back
-to 0x0 (the BL1 entry point address) on CPU #0. The ``PRELOADED_BL33_BASE`` define
-used when compiling the FIP must match the ELF entry point.
-
-Boot of a preloaded bootwrapped kernel image on Juno
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-The procedure to obtain and compile the boot wrapper is very similar to the case
-of the FVP. The execution must be stopped at the end of bl2\_main(), and the
-loading method explained above in the EL3 payload boot flow section may be used
-to load the ELF file over JTAG on Juno.
+    -C pctl.startup=0.0.0.0                                     \
+    -C bp.secure_memory=1                                       \
+    -C cluster0.NUM_CORES=4                                     \
+    -C cluster1.NUM_CORES=4                                     \
+    -C cache_state_modelled=1                                   \
+    -C cluster0.cpu0.RVBAR=0x04020000                           \
+    -C cluster0.cpu1.RVBAR=0x04020000                           \
+    -C cluster0.cpu2.RVBAR=0x04020000                           \
+    -C cluster0.cpu3.RVBAR=0x04020000                           \
+    -C cluster1.cpu0.RVBAR=0x04020000                           \
+    -C cluster1.cpu1.RVBAR=0x04020000                           \
+    -C cluster1.cpu2.RVBAR=0x04020000                           \
+    -C cluster1.cpu3.RVBAR=0x04020000                           \
+    --data cluster0.cpu0="<path-to>/bl31.bin"@0x04020000        \
+    --data cluster0.cpu0="<path-to>/<patched-fdt>"@0x82000000   \
+    --data cluster0.cpu0="<path-to>/<kernel-binary>"@0x80080000 \
+    --data cluster0.cpu0="<path-to>/<ramdisk.img>"@0x84000000
+
+Boot of a preloaded kernel image on Juno
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The Trusted Firmware must be compiled in a similar way as for FVP explained
+above. The process to load binaries to memory is the one explained in
+`Booting an EL3 payload on Juno`_.
 
 Running the software on FVP
 ---------------------------