stm32mp: stm32prog: Add support of ESP partition type

Add support of "ESP" partition type in flashlayout to select
the "EFI System Partition", associated to U-Boot "system"
partition type  guid, PARTITION_SYSTEM_GUID =
C12A7328-F81F-11d2-BA4B-00A0C93EC93B.

This partition is the bootable partition for efi boot.

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