opendingux: Wait until udev is done before accessing /usr/local

/usr/local will be a symlink to /media/data/local, so we have to
make sure that /media/data is mounted by udev first.
opendingux: Make /usr/local point to /media/data/local
opendingux: Remount /boot read-only, not read-write
opendingux: Don't try to mount /media/system