Move TARGET_RELEASETOOLS_EXTENSIONS from BoardConfigCommon.mk

doing this for tilapia

Change-Id: I07f65a49ce28ba8f18c0b3ef409c6fe639170dbf
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 232a207..22cd20e 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -35,5 +35,7 @@
 
 TARGET_RECOVERY_UI_LIB := librecovery_ui_grouper
 
+TARGET_RELEASETOOLS_EXTENSIONS := device/asus/grouper
+
 -include vendor/asus/grouper/BoardConfigVendor.mk
 include device/asus/grouper/BoardConfigCommon.mk