soong_config: Support SYSTEM_EXT* sepolicy vars

The new variable name reflects its actual usage.

Keep compatibility with BOARD_PLAT_* because it has been a
convention for years. Also add warning messages for BOARD_PLAT_*
variables via KATI_deprecated_var.

Test: `make selinux_policy` with
      `SYSTEM_EXT_{PUBLIC,PRIVATE_SEPOLICY_DIRS}` set,
      observe additions in `$(TARGET_COPY_OUT_SYSTEM_EXT)/etc/selinux`

Signed-off-by: Felix Elsner <google@ix5.org>
Change-Id: I58c64839cc513ae082cd3ee3c1e108843ea7439e
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 5b83aa5..d5e16f7 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -174,8 +174,9 @@
 $(call add_json_list, BoardOdmSepolicyDirs,              $(BOARD_ODM_SEPOLICY_DIRS))
 $(call add_json_list, BoardVendorDlkmSepolicyDirs,       $(BOARD_VENDOR_DLKM_SEPOLICY_DIRS))
 $(call add_json_list, BoardOdmDlkmSepolicyDirs,          $(BOARD_ODM_DLKM_SEPOLICY_DIRS))
-$(call add_json_list, SystemExtPublicSepolicyDirs,       $(BOARD_PLAT_PUBLIC_SEPOLICY_DIR))
-$(call add_json_list, SystemExtPrivateSepolicyDirs,      $(BOARD_PLAT_PRIVATE_SEPOLICY_DIR))
+# TODO: BOARD_PLAT_* dirs only kept for compatibility reasons. Will be a hard error on API level 31
+$(call add_json_list, SystemExtPublicSepolicyDirs,       $(SYSTEM_EXT_PUBLIC_SEPOLICY_DIRS) $(BOARD_PLAT_PUBLIC_SEPOLICY_DIR))
+$(call add_json_list, SystemExtPrivateSepolicyDirs,      $(SYSTEM_EXT_PRIVATE_SEPOLICY_DIRS) $(BOARD_PLAT_PRIVATE_SEPOLICY_DIR))
 $(call add_json_list, BoardSepolicyM4Defs,               $(BOARD_SEPOLICY_M4DEFS))
 
 $(call add_json_bool, Flatten_apex,                      $(filter true,$(TARGET_FLATTEN_APEX)))