Android.mk: Add support for BOARD_SEPOLICY_M4DEFS

Allow device builders to pass arbitrary m4 definitions
during the build via make variable BOARD_SEPOLICY_M4DEFS.
This enables OEMs to define their own static policy build
conditionals.

Change-Id: Ibea1dbb7b8615576c5668e47f16ed0eedfa0b73c
Signed-off-by: William Roberts <william.c.roberts@intel.com>