rules.mak: Add "COMMA" constant

Using "," literal in $(call quiet-command, ...) arguments is awkward.
Add this constant to make it at least doable.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1464755128-32490-3-git-send-email-famz@redhat.com
diff --git a/rules.mak b/rules.mak
index d1ff311..4a8f464 100644
--- a/rules.mak
+++ b/rules.mak
@@ -1,4 +1,6 @@
 
+COMMA := ,
+
 # Don't use implicit rules or variables
 # we have explicit rules for everything
 MAKEFLAGS += -rR