Do not initialize EXTRA_SNAPS with an empty value. Allow overrides via the environment.

debian/changelog

@@ -3,7 +3,11 @@ livecd-rootfs (2.609) UNRELEASED; urgency=medium
   [ Ethan Hsieh ]
   * Add support for the Nitrogen6x board (LP: #1838064).
 
- -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Thu, 05 Sep 2019 17:29:57 +0200
+  [ Łukasz 'sil2100' Zemczak ]
+  * Do not initialize EXTRA_SNAPS with an empty value. Allow overrides via the
+    environment.
+
+ -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Fri, 06 Sep 2019 12:06:51 +0200
 
 livecd-rootfs (2.608) eoan; urgency=medium
 

live-build/auto/config

@@ -342,7 +342,6 @@ case $IMAGEFORMAT in
 
 		if [ $PROJECT = "ubuntu-core" ]; then
 			# snap-based core images
-			EXTRA_SNAPS=""
 
 			case $MODEL in
 				pc-amd64|pc-i386)
@@ -362,7 +361,7 @@ case $IMAGEFORMAT in
 
 			case "$ARCH+${SUBARCH:-}" in
 				amd64+kassel)
-					EXTRA_SNAPS="core bluez alsa-utils"
+					EXTRA_SNAPS="$EXTRA_SNAPS core bluez alsa-utils"
 					;;
 				*) ;;
 			esac