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/samsung_slsi/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