init: Allow first-stage mounts in the second stage if they're formattable.

If /metadata is wiped, but is specified as a first-stage mount, it'll
fail to mount and then skipped as a mount in the second stage. Rather
than add a new flag, this patch piggy-backs on "formattable" since
otherwise there is no way the file system will be formatted.

Bug: 121209697
Test: flashall -w && reboot
Change-Id: If36a12251f398a99b9423713a8bfbe8c33523b66