Inform auto/build that we're using ubuntu-image.

live-build/auto/config

@@ -481,7 +481,6 @@ case $IMAGEFORMAT in
 			for snap in $EXTRA_SNAPS; do
 				UBUNTU_IMAGE_ARGS="$UBUNTU_IMAGE_ARGS --extra-snaps $snap"
 			done
-			echo "IMAGEFORMAT=$IMAGEFORMAT" >> config/common
 			echo "UBUNTU_IMAGE_ARGS=\"$UBUNTU_IMAGE_ARGS\"" >> config/common
 			# Store model assertion in top dir to get it picked up later as a build artifact
 			env SNAPPY_STORE_NO_CDN=1 snap known --remote model series=16 model="$MODEL" brand-id=canonical > "$PREFIX".model-assertion
@@ -504,6 +503,9 @@ case $IMAGEFORMAT in
 
 		# Save the model name used for building, mostly for any model-specific hook execution
 		echo "MODEL=$MODEL" >> config/common
+		# Inform auto/build that we're using ubuntu-image
+		echo "IMAGEFORMAT=$IMAGEFORMAT" >> config/common
+
 		# Fake finished configuration for lb build
 		mkdir -p .build
 		touch .build/config