snap preseeding: capture Ubuntu snap seeding policy in code

When no channel is given for a pre-seeded snap, the channel
should be stable/ubuntu-XX.YY to match Ubuntu policy.

This patch adds the LB_DISTRIBUTION var to config/{binary,chroot}
to have a common way of determining the Ubuntu XX.YY version
number between hooks and auto/build.

live-build/auto/build

@@ -39,10 +39,8 @@ preinstall_snaps() {
 	lb chroot_resolv install
 	snap_prepare chroot
     
-	snap_channel="stable/ubuntu-$(distro-info --series="$LB_DISTRIBUTION" \
-	                              -r | awk '{ print $1 }')"
 	for snap in "$@"; do
-		snap_preseed chroot "${snap}" "$snap_channel"
+		snap_preseed chroot "${snap}"
 	done
 	lb chroot_resolv remove
 }

live-build/auto/config

@@ -825,9 +825,11 @@ lb config noauto \
 
 echo "LB_CHROOT_HOOKS=\"$CHROOT_HOOKS\"" >> config/chroot
 echo "SUBPROJECT=\"${SUBPROJECT:-}\"" >> config/chroot
+echo "LB_DISTRIBUTION=\"$SUITE\"" >> config/chroot
 echo "LB_BINARY_HOOKS=\"$BINARY_HOOKS\"" >> config/binary
 echo "BUILDSTAMP=\"$NOW\"" >> config/binary
 echo "SUBPROJECT=\"${SUBPROJECT:-}\"" >> config/binary
+echo "LB_DISTRIBUTION=\"$SUITE\"" >> config/binary
 
 case $ARCH+$SUBARCH in
 	armhf+raspi2)

live-build/functions

@@ -362,6 +362,11 @@ recreate_initramfs() {
 	mv "$CHROOT"/boot/initrd.img-* $DESTDIR
 }
 
+release_ver() {
+    # Return the release version number
+    distro-info --series="$LB_DISTRIBUTION" -r | awk '{ print $1 }'
+}
+
 _snap_preseed() {
     # Download the snap/assertion and add to the preseed
     local CHROOT_ROOT=$1
@@ -442,7 +447,9 @@ snap_preseed() {
     # Preseeed a snap in the image
     local CHROOT_ROOT=$1
     local SNAP=$2
-    local CHANNEL=${3:-stable}
+    # Per Ubuntu policy, all seeded snaps (with the exception of the core
+    # snap) must pull from stable/ubuntu-$(release_ver) as their channel.
+    local CHANNEL=${3:-"stable/ubuntu-$(release_ver)"}
 
     snap_prepare $CHROOT_ROOT
     _snap_preseed $CHROOT_ROOT $SNAP $CHANNEL