vendor: omni: Preserve device definition for WITH_DEXPREOPT

Respect the device definition before set global/default behaviour
for Nightlies/Weeklies/Security builds.

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I864a963f5a2761caa040f23d4e7d993f271e452d
diff --git a/config/common.mk b/config/common.mk
index 7a90cb3..572d9f5 100644
--- a/config/common.mk
+++ b/config/common.mk
@@ -86,15 +86,21 @@
 
 # Enable dexpreopt for all nightlies
 ifeq ($(ROM_BUILDTYPE),NIGHTLY)
-    WITH_DEXPREOPT := true
+    ifeq ($(WITH_DEXPREOPT),)
+        WITH_DEXPREOPT := true
+    endif
 endif
 # and weeklies
 ifeq ($(ROM_BUILDTYPE),WEEKLY)
-    WITH_DEXPREOPT := true
+    ifeq ($(WITH_DEXPREOPT),)
+        WITH_DEXPREOPT := true
+    endif
 endif
 # and security releases
 ifeq ($(ROM_BUILDTYPE),SECURITY_RELEASE)
-    WITH_DEXPREOPT := true
+    ifeq ($(WITH_DEXPREOPT),)
+        WITH_DEXPREOPT := true
+    endif
 endif
 # but not homemades
 ifeq ($(ROM_BUILDTYPE),HOMEMADE)