Board Support for the stm32mp1 platform is contained in the device tree,
so if we remove hardcoding of board name from the Makefile, we can build
the intermediary objects once and generate one new tf-a-*.stm32 binary
for every device tree specified. All in one go.

With implicit rules implemented, we only need to change the top level
target to support multi-image builds on the stm32mp1.

Change-Id: I4cae7d32a4c03a3c29c559dc5332e002223902c1
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>