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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>