Allow restoring to read-only file system

This allows the restoration of firmware or modems on read-only
mounted file systems. If the user is able to make a backup of
these partitions, it would only make sense they be allowed to
restore that backup. The current handling is dangerous in that
it happily wipes, but refuses to restore. :)

Note that it would be preferable to use the backup=emmc flag
instead in most cases, added by change:
https://gerrit.omnirom.org/#/c/17183

Change-Id: I32d47c8928dee61595c15a9db16d3c5b9a6d7183