generate_medkit: allow no branch specification

There should be default value set for branch so no branch option
requirement.

generate_medkit

@@ -17,7 +17,7 @@
 set -e
 
 export BOARD=
-export BRANCH=
+export BRANCH="hbd"
 export L10N=en,cs
 # TODO fill in default lists when we have them selected
 export LISTS=
@@ -117,10 +117,6 @@ done
 	echo "You have to specify target Turris router." >&2
 	exit 1
 }
-[ -n "$BRANCH" ] || {
-	echo "You have to specify target branch." >&2
-	exit 1
-}
 [ -n "$OUTPUT" ] || {
 	case "$BOARD" in
 		mox)