vendor: clean up versioning

there is no need to define these variables twice.

if the build type must be something other than "HOMEMADE",
include "export ROM_BUILDTYPE=build_type" with the make command

and fix the predexopt derp

Change-Id: If698faa90be5a3367183919412c3fa0745e99f63
diff --git a/config/common.mk b/config/common.mk
index 2dff26e..8ba671a 100644
--- a/config/common.mk
+++ b/config/common.mk
@@ -74,5 +74,5 @@
 
 # Enable dexpreopt for all nightlies
 ifeq ($(ROM_BUILDTYPE),NIGHTLY)
-    WITH_DEXPREOPT := TRUE
+    WITH_DEXPREOPT := true
 endif