mkimage: Add support for bundling TFA BL31 in mkimage -f auto

mkimage: Add support for bundling TFA BL31 in mkimage -f auto

Introduce two new parameters to be used with mkimage -f auto to bundle
TFA BL31 image into fitImage, using auto-generated fitImage. Add -y to
specify TFA BL31 file name and -Y to specify TFA BL31 load and entry
point address. This is meant to be used with systems which boot all of
TFA BL31, Linux and its DT from a single fitImage, all booted by U-Boot.

Example invocation:
"
$ mkimage -E -A arm64 -C none -e 0x50200000 -a 0x50200000 -f auto \
          -d arch/arm64/boot/Image \
	  -b arch/arm64/boot/dts/renesas/r8a779g3-sparrow-hawk.dtb \
	  -y ../tfa/build/rcar_gen4/release/bl31.bin -Y 0x46400000 \
	  /path/to/output/fitImage
"

Documentation update and test are also included, the test validates
both positive and negative test cases, where fitImage does not include
TFA BL31 and does include TFA BL31 blobs.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>