Add skip_themes directive to build process

Change-Id: I8fad4ebe3998c41ce93be2e3127552ad975ff510
diff --git a/products/themes.mk b/products/themes.mk
index 39d6ffa..00d1045 100644
--- a/products/themes.mk
+++ b/products/themes.mk
@@ -1,4 +1,7 @@
 # Theme packages
+
+ifndef SKIP_THEMES
 PRODUCT_PACKAGES += \
        Androidian \
        Cyanbread
+endif
\ No newline at end of file