stm32mp1: use a specific SD/eMMC partition for U-Boot enviromnent

stm32mp1: use a specific SD/eMMC partition for U-Boot enviromnent

Save the environment at the end of the U-Boot partition, the GPT
partition named "ssbl" in SD card or eMMC and avoid requirements
on the "bootfs" file system generated via specific raw tools
(like wic and genimage).

With the previous configuration of the U-Boot environment saved in ext4
file, U-Boot need to create/modify the file uenv.txt in the ext4 file
system; so this EXT4 file system need to be generated without some
functionality, like metadata_csum and dir_index, because they are not
supported by U-Boot.

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