Added 'clean' task to Makefile.

Author: mikeller

Makefile

@@ -1,7 +1,10 @@
 all:
 	bin/build.sh
 
-release: all
+clean:
+	rm -rf obj/*
+
+release: clean all
 	FILE_NAME="betaflight-tx-lua-scripts_$$(git describe --abbrev=0 --tags).zip"; \
 	rm -f $${FILE_NAME}; \
 	zip -r $${FILE_NAME} obj/