refactor(plat/marvell/a3k): Rename DOIMAGETOOL to TBB
refactor(plat/marvell/a3k): Rename DOIMAGETOOL to TBB

Armada 3700 uses external TBB tool for creating images and does not use
internal TF-A doimage tool from tools/marvell/doimage/

Therefore set correct name of variable.

Signed-off-by: Pali Rohár <84fc5b246d98b743737687ddff985e1cb7a72c1e@kernel.org>
Change-Id: I38a94dca78d483de4c79da597c032e1e5d06d92d

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware