Re-add brand variable

Accidentally dropped in change If1a7835aca0832e41e664caed373707ce711f08a

Change-Id: I12857ee33b95adb30b8ad347ab277c2acc221657
diff --git a/config/common.mk b/config/common.mk
index 61abb70..de6a84c 100644
--- a/config/common.mk
+++ b/config/common.mk
@@ -1,3 +1,5 @@
+PRODUCT_BRAND ?= cyanogenmod
+
 ifdef CM_NIGHTLY
 PRODUCT_PROPERTY_OVERRIDES += \
     ro.rommanager.developerid=cyanogenmodnightly