Separate system_ext_sepolicy.cil out of system sepolicy

Currently system sepolicy can be extended via:
  - BOARD_PLAT_PUBLIC_SEPOLICY_DIR
  - BOARD_PLAT_PRIVATE_SEPOLICY_DIR

To support having a single shared core system sepolicy, those
customization should be moved to the newly added system_ext partition.

System-ext-specific sepolicy files will be installed into
/system_ext/etc/selinux/*. system_ext_sepolicy.cil is merged into
precompiled_sepolicy at build-time. In case precompiled_sepolicy can't
be used (e.g. system-only-ota), the init will merge this file with
the rest of the sepolicy at runtime.

Bug: 137712473
Test: boot aosp_crosshatch with system_ext_sepolicy.cil
Test: boot aosp_crosshatch without system_ext_sepolicy.cil
Change-Id: I53984e8a973eff15de0653916203020dbabe9c03
diff --git a/compat.mk b/compat.mk
index 30904ef..9604403 100644
--- a/compat.mk
+++ b/compat.mk
@@ -18,6 +18,13 @@
     $(built_vendor_cil) \
     $(ALL_MODULES.$(version).compat.cil.BUILT) \
 
+ifdef HAS_SYSTEM_EXT_SEPOLICY
+all_cil_files += \
+    $(built_system_ext_cil) \
+    $(built_system_ext_mapping_cil) \
+
+endif
+
 ifdef HAS_PRODUCT_SEPOLICY
 all_cil_files += \
     $(built_product_cil) \