SELinux policy for MediaTek devices

Don't recurse into the platform makefiles. We don't care about them, and we don't want to force a reset of BOARD_SEPOLICY_DIRS.

If you want to use these policies, add a

include device/mediatek/sepolicy/sepolicy.mk

to your device's BoardConfig. It is highly recommended that in case you have your own BOARD_SEPOLICY_DIRS declaration, the inclusion happens before those lines