recovery: make /etc/fstab only include entries that match the detected fs type

 * toybox's `mount` does not support multiple entries like we do,
   so if we can detect the filesystem of an fstab enrtry print that
   and only that one to /etc/fstab, so that mounting via toybox has
   a better chance of succeding.

 * as a bonus, this patch also gets rid of duplicates in /etc/fstab
   caused by the fact that ReadDefaultFstab() combines entries from
   DT and from recovery.fstab

Change-Id: Iec4ab38044054555d2a33da6f5d53de7716e7bee