[buildroot] Add two debug rules the examine the values of runtime make variables.

Based on patch by Philip Prindeville <philipp@redfish-solutions.com>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@23389 3c298f89-4303-0410-b956-a3cf2f4a3e73

rules.mk

@@ -254,4 +254,16 @@ all:
 FORCE: ;
 .PHONY: FORCE
 
+val.%:
+	@$(if $(filter undefined,$(origin $*)),\
+		echo "$* undefined" >&2, \
+		echo '$(subst ','"'"',$($*))' \
+	)
+
+var.%:
+	@$(if $(filter undefined,$(origin $*)),\
+		echo "$* undefined" >&2, \
+		echo "$*='"'$(subst ','"'\"'\"'"',$($*))'"'" \
+	)
+
 endif #__rules_inc