stm32mp: stm32prog: Add support of ENV partition type

Add support of "ENV" partition type in flashlayout to select
the "u-boot-env" GUID, with PARTITION_U_BOOT_ENVIRONMENT =
3de21764-95bd-54bd-a5c3-4abe786f38a8, that mean a partition
holding a U-Boot environment introduced by
commit c0364ce1 ("doc/README.gpt: define partition type
GUID for U-Boot environment")'

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