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>
diff --git a/plat/marvell/armada/common/mss/mss_common.mk b/plat/marvell/armada/common/mss/mss_common.mk
index 050d88d..4ab4359 100644
--- a/plat/marvell/armada/common/mss/mss_common.mk
+++ b/plat/marvell/armada/common/mss/mss_common.mk
@@ -13,7 +13,7 @@
 				$(PLAT_MARVELL)/common/plat_delay_timer.c	\
 				drivers/delay_timer/delay_timer.c		\
 				$(MARVELL_DRV)					\
-				$(PLAT_FAMILY_BASE)/$(PLAT)/board/marvell_plat_config.c
+				$(BOARD_DIR)/board/marvell_plat_config.c
 
 BL31_SOURCES		+=	$(MSS_SOURCE)/mss_ipc_drv.c