Allow custom model assertions in snap_prepare_assertions

This also splits up the preparation in to two functions, so that images that
want to use a custom model assertion but don't have any snaps to preinstall
don't end up with the core snap installed.

live-build/functions

@@ -412,9 +412,12 @@ EOF
     (cd $snaps_dir; ls -1 ${SNAP_NAME}_*.snap) >> $seed_yaml
 }
 
-snap_prepare() {
-    # Configure basic snapd assertions and pre-seeds the 'core' snap
+snap_prepare_assertions() {
+    # Configure basic snapd assertions
     local CHROOT_ROOT=$1
+    # Optional. If set, should be a colon-separated string of brand:model to be
+    # used for the image's model assertion
+    local CUSTOM_BRAND_MODEL=$2
 
     local seed_dir="$CHROOT_ROOT/var/lib/snapd/seed"
     local snaps_dir="$seed_dir/snaps"
@@ -426,9 +429,16 @@ snap_prepare() {
     mkdir -p "$assertions_dir"
     mkdir -p "$snaps_dir"
 
+    local brand="generic"
+    local model="generic-classic"
+    if [ -n "$CUSTOM_BRAND_MODEL" ] ; then
+        brand="$(echo $CUSTOM_BRAND_MODEL | cut -d: -f 1)"
+        brand="$(echo $CUSTOM_BRAND_MODEL | cut -d: -f 2)"
+    fi
+
     if ! [ -e "$model_assertion" ] ; then
         snap known --remote model series=16 \
-            model=generic-classic brand-id=generic \
+            model=$model brand-id=$brand \
             > "$model_assertion"
     fi
 
@@ -446,6 +456,16 @@ snap_prepare() {
         snap known --remote account account-id=$account \
             > "$account_assertion"
     fi
+}
+
+snap_prepare() {
+    # Configure basic snapd assertions and pre-seeds the 'core' snap
+    local CHROOT_ROOT=$1
+    # Optional. If set, should be a colon-separated string of brand:model to be
+    # used for the image's model assertion
+    local CUSTOM_BRAND_MODEL=$2
+
+    snap_prepare_assertions "$CHROOT_ROOT" "$CUSTOM_BRAND_MODEL"
 
     # Download the core snap
     if ! [ -f $snaps_dir/core_[0-9]*.snap ] ; then