recovery: nandroid: implement preference for backup_format

ro.cwm.prefer.backup_format takes:
dup (dedupe)
tgz (tar + gzip)
(i.e. setprop ro.cwm.prefer.backup_format tgz)

-If nothing is specified or the above is not matched then tar is used.
-If the user specifies a default_backup_format then that is used.

Change-Id: I935d6cc732c5ef2b1d0d44f5459425e85702e710