common: Set property for the CM releasetype

Set ro.cm.releasetype based on the deteremined releasetype of the
build.

Change-Id: I2c1aa7181f42342594bd6d5dc5f5252220ed3497
diff --git a/config/common.mk b/config/common.mk
index af67cf5..20138e1 100644
--- a/config/common.mk
+++ b/config/common.mk
@@ -301,6 +301,7 @@
 
 PRODUCT_PROPERTY_OVERRIDES += \
   ro.cm.version=$(CM_VERSION) \
+  ro.cm.releasetype=$(CM_BUILDTYPE) \
   ro.modversion=$(CM_VERSION) \
   ro.cmlegal.url=http://www.cyanogenmod.org/docs/privacy