Fix "Add skip_themes directive to build process"

This reverts commit 090e2ec0c1e576bc12be069ce7991eace4d3c9d9.

Change-Id: I8a6e85b1fd356eeef4572c861c6f8d72fb44de97
diff --git a/products/themes.mk b/products/themes.mk
index 00d1045..856ab1f 100644
--- a/products/themes.mk
+++ b/products/themes.mk
@@ -1,7 +1,7 @@
 # Theme packages
 
-ifndef SKIP_THEMES
+ifneq ($(SKIP_THEMES),true)
 PRODUCT_PACKAGES += \
        Androidian \
        Cyanbread
-endif
\ No newline at end of file
+endif