configs/stm32mp157*: switch to use TF-A based configurations

configs/stm32mp157*: switch to use TF-A based configurations

This commit changes the stm32mp1 configurations to use the TF-A ->
U-Boot boot flow (called "trusted boot flow" by ST) instead of the
U-Boot SPL -> U-Boot boot flow. ST recommands this trusted boot, and
it's also the only one that allows to use the STM32 Cube Programmer
for reflashing devices.

As part of this, the genimage configuration file is now generated, as
it contains some file names that depend on the platform. Removing
partition-type from genimage.cfg is mandatory, since it would cause a
hybrid MBR partition table to be created, which itself would prevent
ATF from finding the ssbl parition.

Signed-off-by: Shlomi Vaknin <shlomi.39sd@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>