refactor(st): move FWU support to common code

Move PLAT_PARTITION_MAX_ENTRIES and all other definitions linked to it
to common.mk.
Move drivers/fwu/fwu.mk inclusion there as well.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I5dde65e41908d706328cb8929582f827ceeff841
3 files changed