Doc: Add the "Building the Test Secure Payload" section

Add a section in the user guide explaining how to compile the TSP
image and include it into the FIP. This includes instructions to make
the TSP run from Trusted DRAM (rather than Trusted SRAM) on FVP.

Change-Id: I04780757a149eeb5482a12a61e821be947b882c0

docs/user-guide.md

@@ -89,7 +89,8 @@ To build the software for the FVPs, follow these steps:
     By default this produces a release version of the build. To produce a debug
     version instead, refer to the "Debugging options" section below. UEFI can be
     used as the BL3-3 image, refer to the "Obtaining the normal world software"
-    section below.
+    section below. By default this won't compile the TSP in, refer to the
+    "Building the Test Secure Payload" section below.
 
     The build process creates products in a `build` directory tree, building
     the objects and binaries for each boot loader stage in separate
@@ -243,6 +244,48 @@ Extra debug options can be passed to the build system by setting `CFLAGS`:
 NOTE: The Foundation FVP does not provide a debugger interface.
 
 
+### Building the Test Secure Payload
+
+The TSP is coupled with a companion runtime service in the BL3-1 firmware,
+called the TSPD. Therefore, if you intend to use the TSP, the BL3-1 image
+must be recompiled as well. For more information on SPs and SPDs, see the
+"Secure-EL1 Payloads and Dispatchers" section in the [Firmware Design].
+
+First clean the Trusted Firmware build directory to get rid of any previous
+BL3-1 binary. Then to build the TSP image and include it into the FIP use:
+
+    CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-none-elf- \
+    BL33=<path-to>/<bl33_image>                               \
+    make PLAT=fvp SPD=tspd all fip
+
+An additional boot loader binary file is created in the `build` directory:
+
+    *   `build/<platform>/<build-type>/bl32.bin`
+
+The Firmware Package contains this new image:
+
+    Firmware Image Package ToC:
+    ---------------------------
+    - Trusted Boot Firmware BL2: offset=0xD8, size=0x6000
+      file: './build/fvp/release/bl2.bin'
+    - EL3 Runtime Firmware BL3-1: offset=0x60D8, size=0x9000
+      file: './build/fvp/release/bl31.bin'
+    - Secure Payload BL3-2 (Trusted OS): offset=0xF0D8, size=0x3000
+      file: './build/fvp/release/bl32.bin'
+    - Non-Trusted Firmware BL3-3: offset=0x120D8, size=0x280000
+      file: '../FVP_AARCH64_EFI.fd'
+    ---------------------------
+    Creating "build/fvp/release/fip.bin"
+
+On FVP, the TSP binary runs from Trusted SRAM by default. It is also possible
+to run it from Trusted DRAM. This is controlled by the build configuration
+`TSP_RAM_LOCATION`:
+
+    CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-none-elf- \
+    BL33=<path-to>/<bl33_image>                               \
+    make PLAT=fvp SPD=tspd TSP_RAM_LOCATION=tdram all fip
+
+
 ### Checking source code style
 
 When making changes to the source for submission to the project, the source