Allow buildtype user

The tools scripts did not handle buildtype user correctly.

Change-Id: Ie4f98dbab7e97a8244ee1d9b4d995673e0aac930
diff --git a/tools/opticharger b/tools/opticharger
index 33489d8..1207b6b 100755
--- a/tools/opticharger
+++ b/tools/opticharger
@@ -32,6 +32,8 @@
 then
     if [ "$TARGET_BUILD_VARIANT" = "userdebug" ]; then
        TARGET_BUILD_VARIANT="eng"
+    elif [ "$TARGET_BUILD_VARIANT" = "user" ]; then
+       TARGET_BUILD_VARIANT="eng"
     fi
 
     APKCERTS=$OUT/obj/PACKAGING/target_files_intermediates/$TARGET_PRODUCT-target_files-$TARGET_BUILD_VARIANT.$USER/META/apkcerts.txt