plat: marvell: armada: make a8k_common.mk and mss_common.mk more generic

plat: marvell: armada: make a8k_common.mk and mss_common.mk more generic

As a preparation for upcoming support for CN9130 platform, which is
classified as OcteonTx2 product but inherits functionality from a8k,
allow to use a8k_common.mk and mss_common.mk from outside of
PLAT_FAMILY_BASE.
Above is done by introducing BOARD_DIR which needs to be set by each
platform, before including a8k_common.mk and mss_common.mk. This will
allow to use mentioned mk files not only for platforms located under
previously defined PLAT_FAMILY_BASE.

Change-Id: I22356c99bc0419a40ae11e42f37acd50943ea134
Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>