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 - Clarify the documentation of the 'FWU_SMC_IMAGE_COPY' SMC in the
   Firmware Update guide. Also extend the list of pre-conditions to
   include the additional input validation implemented by previous
   patches.

 - Improve documentation of bl1_plat_mem_check() in the porting
   guide. It now specifies that the generic FWU code protects
   bl1_plat_mem_check() from integer overflows resulting from
   the addition of the base address and size passed in arguments.

Change-Id: I07b47a3778df7b9c089529b2dd2135707640a91c
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

Change-Id: I4fcdb8e813e0392c2cd3d0623698e8319b3b0593

This patch adds design documentation for the Firmware Update (FWU)
feature in `firmware-update.md`. It provides an overview of FWU,
describes the BL1 SMC interface, and includes diagrams showing
an example FWU boot flow and the FWU state machine.

This patch also updates the existing TF documents where needed:

*   `porting-guide.md`
*   `user-guide.md`
*   `firmware-design.md`
*   `rt-svc-writers-guide.md`
*   `trusted_board_boot.md`

Change-Id: Ie6de31544429b18f01327bd763175e218299a4ce
Co-Authored-By: Dan Handley <dan.handley@arm.com>