board/stmicroelectronics/common/stm32mp157: fix U-Boot corruption on the first boot

board/stmicroelectronics/common/stm32mp157: fix U-Boot corruption on the first boot

The stm32mp157 images successfully boot only once. Subsequent boot
attempts fail to reach U-Boot. The root cause turns out to be in U-Boot
corruption during the first boot. The stm32mp1 U-Boot stores its
environment at the end of GPT partition named 'ssbl' since v2020.10-rc2.
However Buildroot genimage template for stm32mp157 boards creates 'ssbl'
partition w/o extra space for U-Boot primary and redundant environments.
This patch explicitly specifies 'ssbl' partition size that should be
enough for both u-boot.stm32 image (~1M) and both environments (16K).

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
Tested-by: Pierre-Jean Texier <texier.pj2@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>