Add ro.cm.device to build.prop

As requested by Ricardo Cerqueira:
http://r.cyanogenmod.com/#change,11423,patchset=1

Change-Id: If08cc13c0f0e92e0dd262a2f71610070a54aaead
diff --git a/tools/buildinfo.sh b/tools/buildinfo.sh
index 5c199b8..310d478 100755
--- a/tools/buildinfo.sh
+++ b/tools/buildinfo.sh
@@ -56,4 +56,6 @@
 fi
 echo "ro.build.characteristics=$TARGET_AAPT_CHARACTERISTICS"
 
+echo "ro.cm.device=$CM_DEVICE"
+
 echo "# end build properties"