Use more generic variable for not keeping prepared image

Use this:
DISCARD_PREPARED_IMAGE=1

Makefile

@@ -16,7 +16,7 @@ endif
 TEMPLATE_ENV_WHITELIST += \
 	DIST DISTRIBUTION TEMPLATE_SCRIPTS TEMPLATE_NAME TEMPLATE_FLAVOR \
 	TEMPLATE_FLAVOR_DIR TEMPLATE_OPTIONS APPMENUS_DIR \
-	VERBOSE DEBUG PATH BUILDER_DIR SRC_DIR \
+	VERBOSE DEBUG PATH BUILDER_DIR SRC_DIR DISCARD_PREPARED_IMAGE \
 	TEMPLATE_ROOT_WITH_PARTITIONS USE_QUBES_REPO_VERSION \
 	USE_QUBES_REPO_TESTING BUILDER_TURBO_MODE REPO_PROXY
 

qubeize_image

@@ -61,8 +61,8 @@ trap cleanup ERR
 export IMG="qubeized_images/$NAME/root.img"
 mkdir -p "qubeized_images/$NAME"
 
-if [ "$TRAVIS" = "true" ]; then
-    echo "--> Moving $CLEANIMG to $IMG (save space on Travis-CI)..."
+if [ "0$DISCARD_PREPARED_IMAGE" -eq "1" ]; then
+    echo "--> Moving $CLEANIMG to $IMG..."
     mv "$CLEANIMG" "$IMG" || exit 1
 else
     echo "--> Copying $CLEANIMG to $IMG..."