plat: marvell: armada: Building ${DOIMAGETOOL} is only for a8k

plat: marvell: armada: Building ${DOIMAGETOOL} is only for a8k

Currently a3k target is misusing ${DOIMAGETOOL} target for building flash
and UART images. It is not used for building image tool.

So move ${DOIMAGETOOL} target from common marvell include file into a8k
include file and add correct invocation of ${MAKE} into a3k for building
flash and UART images.

Part of this change is also checks that MV_DDR_PATH for a3k was specified
by user as this option is required for building a3k flash and UART images.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I5ae9d08b8505460933f17836c9b6435fd6e51bb6