build: small improvement for cantrip-builtins-clean*

Change-Id: I713a5f4868b4aa93564838721002d94c60cae239

cantrip_builtins.mk

diff --git a/cantrip_builtins.mk b/cantrip_builtins.mk
index cf19c13..4dd78eb 100644
--- a/cantrip_builtins.mk
+++ b/cantrip_builtins.mk
@@ -79,11 +79,14 @@
 ## Generates both debug & release cpio archives of Cantrip builtins
 cantrip-builtins: cantrip-builtins-debug cantrip-builtins-release
 
+# TODO(sleffler): clean intermedite build artifacts (platform specific)
 cantrip-builtins-clean-debug:
+	rm -rf $(CANTRIP_OUT_DEBUG)/apps
 	rm -rf $(CANTRIP_OUT_DEBUG)/builtins
 	rm -f $(CANTRIP_OUT_DEBUG)/ext_builtins.cpio
 
 cantrip-builtins-clean-release:
+	rm -rf $(CANTRIP_OUT_RELEASE)/apps
 	rm -rf $(CANTRIP_OUT_RELEASE)/builtins
 	rm -f $(CANTRIP_OUT_RELEASE)/ext_builtins.cpio