Address snap base regression after snap-tool removal

With the removal of snap-tool failures are seen in image builds that do
not have the 'core' snap included by the seed.  This is the case for the
minimized subproject of the ubuntu-cpc project where lxd/core is removed.
In that subproject, any binary hook which adds a snap that is based
on 'core' will not add 'core' and fail 'snap debug validate-seed'.
snap-tool included the following logic in the 'snap-tool info' when
determining snap bases:

    # Have "base" initialized to something meaningful.
    if self.is_core_snap():
        snap_data["snap"]["base"] = ""
    elif snap_data["snap"].get("base") is None:
        snap_data["snap"]["base"] = "core"

The snap store does not return a base if the base is core which makes
this necessary.  This patch looks for the base in 'snap info' output
and if none is found (and the snap is not snapd or core) it assumes the
base is 'core' and installs it.  This restores the behavior lost in the
migration from snap-tool to snap cli.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.620.2) eoan; urgency=medium
+
+  * Address snap base regression after snap-tool removal
+
+ -- Robert C Jennings <robert.jennings@canonical.com>  Tue, 25 Feb 2020 16:15:48 -0600
+
 livecd-rootfs (2.620.1) eoan; urgency=medium
 
   * Use snap cli rather than custom snap-tool (LP: #1864252)

live-build/functions

@@ -477,12 +477,16 @@ _snap_preseed() {
         return
     fi
 
+    # Pre-seed snap's base
     case $SNAP_NAME in
         snapd)
             # snapd is self-contained, ignore base
             ;;
+        core|core[0-9][0-9])
+            # core and core## are self-contained, ignore base
+            ;;
         *)
-            # Determine if and what core snap is needed
+            # Determine which core snap is needed
             local snap_info
 
             snap_info=$(snap info --verbose "${SNAP_NAME}")
@@ -494,11 +498,10 @@ _snap_preseed() {
 
             local core_snap=$(echo "$snap_info" | grep '^base:' | awk '{print $2}')
 
-            # If $core_snap is not the empty string then SNAP itself is not a core
-            # snap and we must additionally seed the core snap.
-            if [ -n "$core_snap" ]; then
-                _snap_preseed $CHROOT_ROOT $core_snap stable
-            fi
+            # If snap info does not list a base use 'core'
+            core_snap=${core_snap:-core}
+
+            _snap_preseed $CHROOT_ROOT $core_snap stable
             ;;
     esac