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: NFam Zheng <famz@redhat.com>
Message-id: 1464755128-32490-3-git-send-email-famz@redhat.com

rules.mak

 
+COMMA := ,
+
 # Don't use implicit rules or variables
 # we have explicit rules for everything
 MAKEFLAGS += -rR