update.sh: allow target to be specified

update.sh

 #!/bin/sh
 
+target="${1:-master}"
+
+echo "reset target is $target"
+
 # update meta repo
-git fetch origin master
+git fetch origin "$target"
 # use the latest commit on master, discard any local changes
-git reset --hard origin/master
+git reset --hard "$target"
 # sync repo urls
 git submodule sync --recursive
 # update commits, discard any local changes