Use the BOARD_DIR variable instead of PLAT_FAMILY_BASE variable for
determening the path of the system_power.c file.

The variable was not updated, when it was deprecated in a8k_common.mk
in commit 613bbde.

Signed-off-by: Luka Kovacic <luka.kovacic@sartura.hr>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Change-Id: I9b4659a19ba3cd5c869d44c5d834b220f49136e8