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The common makefile used by every a8k/cn913x platform
(a8k_common.mk) assumed default paths in PLAT_INCLUDES,
BLE/BL31_PORTING_SOURCES. Allow overriding those
variables, in order to avoid code duplication.

It can be helpful in case using multiple board variants
or sharing common settings between different platforms.

Change-Id: Idce603e44ed04d99fb1e3e11a2bb395d552e2bf7
Signed-off-by: Marcin Wojtas <mw@semihalf.com>