add missing target dummy for rx

target/rx/Makefile

@@ -0,0 +1,7 @@
+# This file is part of the OpenADK project. OpenADK is copyrighted
+# material, please see the LICENCE file in the top-level directory.
+
+include $(ADK_TOPDIR)/rules.mk
+include $(ADK_TOPDIR)/mk/kernel-build.mk
+include $(ADK_TOPDIR)/mk/image.mk
+