docs: warp7: Add description for the i.MX7 WaRP7 platform

This patch describes the boot-flow and building of the WaRP7 TF-A port.
What it describes is booting and unsigned TF-A.

A very brief section has been added on signing BL2 which is in no-way
comprehensive. For a comprehensive description of the signing process try
the Boundary Devices blog on the matter.

https://boundarydevices.com/high-assurance-boot-hab-dummies/

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>

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+Trusted Firmware-A for i.MX7 WaRP7
+==================================
+
+The Trusted Firmware-A port for the i.MX7Solo WaRP7 implements BL2 at EL3.
+The i.MX7S contains a BootROM with a High Assurance Boot (HAB) functionality.
+This functionality provides a mechanism for establishing a root-of-trust from
+the reset vector to the command-line in user-space.
+
+Boot Flow
+=========
+
+BootROM --> TF-A BL2 --> BL32(OP-TEE) --> BL33(U-Boot) --> Linux
+
+In the WaRP7 port we encapsulate OP-TEE, DTB and U-Boot into a FIP. This FIP is
+expected and required
+
+# Build Instructions
+
+We need to use a file generated by u-boot in order to generate a .imx image the
+BootROM will boot. It is therefore _required_ to build u-boot before TF-A and
+furthermore it is _recommended_ to use the mkimage in the u-boot/tools directory
+to generate the TF-A .imx image.
+
+## U-Boot:
+
+https://git.linaro.org/landing-teams/working/mbl/u-boot.git
+
+.. code:: shell
+
+    git checkout -b rms-atf-optee-uboot linaro-mbl/rms-atf-optee-uboot
+    make warp7_bl33_defconfig;
+    make u-boot.imx arch=ARM CROSS_COMPILE=arm-linux-gnueabihf-
+
+## TF-A:
+
+https://github.com/ARM-software/arm-trusted-firmware.git
+
+.. code:: shell
+
+    make CROSS_COMPILE=arm-linux-gnueabihf- PLAT=warp7 ARCH=aarch32 ARM_ARCH_MAJOR=7 ARM_CORTEX_A7=yes AARCH32_SP=optee all
+    /path/to/u-boot/tools/mkimage -n /path/to/u-boot/u-boot.cfgout -T imximage -e 0x9df00000 -d ./build/warp7/debug/bl2.bin ./build/warp7/debug/bl2.bin.imx
+
+## OP-TEE:
+
+https://github.com/OP-TEE/optee_os.git
+
+.. code:: shell
+
+    make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- PLATFORM=imx PLATFORM_FLAVOR=mx7swarp7 ARCH=arm CFG_PAGEABLE_ADDR=0 CFG_DT_ADDR=0x83000000 CFG_NS_ENTRY_ADDR=0x87800000
+
+
+## FIP:
+
+.. code:: shell
+
+    mkdir fiptool_images
+    cp /path/to/uboot/u-boot.bin fiptool_images
+    cp /path/to/optee/out/arm-plat-imx/core/tee-header_v2.bin fiptool_images
+    cp /path/to/optee/out/arm-plat-imx/core/tee-pager_v2.bin fiptool_images
+    cp /path/to/optee/out/arm-plat-imx/core/tee-pageable_v2.bin fiptool_images
+    cp /path/to/linux/arch/boot/dts/imx7s-warp.dtb fiptool_images
+    tools/fiptool/fiptool create --tos-fw fiptool_images/tee-header_v2.bin --tos-fw-extra1 fiptool_images/tee-pager_v2.bin --tos-fw-extra2 fiptool_images/tee-pageable_v2.bin --nt-fw fiptool_images/u-boot.bin --hw-config fiptool_images/imx7s-warp.dtb warp7.fip
+
+
+# Deploy Images
+
+
+First place the WaRP7 into UMS mode in u-boot this should produce an entry in
+/dev like /dev/disk/by-id/usb-Linux_UMS_disk_0_WaRP7-0xf42400d3000001d4-0\:0
+
+.. code:: shell
+
+    => ums 0 mmc 0
+
+Next flash bl2.imx and warp7.fip
+
+bl2.imx is flashed @ 1024 bytes
+warp7.fip is flash @ 1048576 bytes
+
+.. code:: shell
+
+    sudo dd if=bl2.bin.imx of=/dev/disk/by-id/usb-Linux_UMS_disk_0_WaRP7-0xf42400d3000001d4-0\:0 bs=512 seek=2 conv=notrunc
+    # Offset is 1MB 1048576 => 1048576 / 512 = 2048
+    sudo dd if=./warp7.fip of=/dev/disk/by-id/usb-Linux_UMS_disk_0_WaRP7-0xf42400d3000001d4-0\:0 bs=512 seek=2048 conv=notrunc
+
+Remember to umount the USB device pefore proceeding
+
+.. code:: shell
+
+    sudo umount /dev/disk/by-id/usb-Linux_UMS_disk_0_WaRP7-0xf42400d3000001d4-0\:0*
+
+
+# Signing BL2
+
+A further step is to sign BL2.
+
+The image_sign.sh and bl2_sign.csf files alluded to blow are available here.
+
+https://github.com/bryanodonoghue/atf-code-signing
+
+It is suggested you use this script plus the example CSF file in order to avoid
+hard-coding data into your CSF files.
+
+Download both "image_sign.sh" and "bl2_sign.csf" to your
+arm-trusted-firmware top-level directory.
+
+.. code:: shell
+
+    #!/bin/bash
+    SIGN=image_sign.sh
+    TEMP=`pwd`/temp
+    BL2_CSF=bl2_sign.csf
+    BL2_IMX=bl2.bin.imx
+    CST_PATH=/path/to/cst-2.3.2
+    CST_BIN=${CST_PATH}/linux64/cst
+
+    #Remove temp
+    rm -rf ${TEMP}
+    mkdir ${TEMP}
+
+    # Generate IMX header
+    /path/to/u-boot/tools/mkimage -n u-boot.cfgout.warp7 -T imximage -e 0x9df00000 -d ./build/warp7/debug/bl2.bin ./build/warp7/debug/bl2.bin.imx > ${TEMP}/${BL2_IMX}.log
+
+    # Copy required items to $TEMP
+    cp build/warp7/debug/bl2.bin.imx ${TEMP}
+    cp ${CST_PATH}/keys/* ${TEMP}
+    cp ${CST_PATH}/crts/* ${TEMP}
+    cp ${BL2_CSF} ${TEMP}
+
+    # Generate signed BL2 image
+    ./${SIGN} image_sign_mbl_binary ${TEMP} ${BL2_CSF} ${BL2_IMX} ${CST_BIN}
+
+    # Copy signed BL2 to top-level directory
+    cp ${TEMP}/${BL2_IMX}-signed .
+    cp ${BL2_RECOVER_CSF} ${TEMP}
+
+
+The resulting bl2.bin.imx-signed can replace bl2.bin.imx in the Deploy
+Images section above, once done.
+
+Suggested flow for verifying.
+
+1. Followed all previous steps above and verify a non-secure ATF boot
+2. Down the NXP Code Singing Tool
+3. Generate keys
+4. Program the fuses on your board
+5. Replace bl2.bin.imx with bl2.bin.imx-signed
+6. Verify inside u-boot that "hab_status" shows no events
+7. Subsequently close your board.
+
+If you have HAB events @ step 6 - do not lock your board.
+
+To get a good over-view of generating keys and programming the fuses on the
+board read "High Assurance Boot for Dummies" by Boundary Devices.
+
+https://boundarydevices.com/high-assurance-boot-hab-dummies/