change autoupdater url
I think it would be easier (especially on the server side) if the autoupdate url would be changed from http://firmware.luebeck.freifunk.net/stable/sysupgrade to http://firmware.luebeck.freifunk.net/stable/images/sysupgrade.
on the server we currently need a symlink like stable -> 0.12.1-1/images/
which is easier to forget than just a link like stable -> 0.12.1-1/
.