board: st: stm32mp2: change bootcmd for ST boards

board: st: stm32mp2: change bootcmd for ST boards

For nor0 boot for the STMicroelectronics boards, the bootfs
is found in SD-Card = mmc0 for nor0 boot.

Introduce a new file configuration file stm32mp25_st_common.h
to manage this specific behavior for the STMicroelectronics
boards; change the boot order for nor0 boot and don't use
the default DISTRO order define in BOOT_TARGET_DEVICES:
mmc1, ubifs, mmc0, mmc2.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Patrice CHOTARD <patrice.chotard@foss.st.com>