wic: Optimise fstab modification for ext2/3/4 and msdos partitions

The fix for [Yocto #13994] required the rootfs directory to be copied
(using hardlinks if possible) when modifying the fstab file under wic.

We can optimise this copy away for filesystems where we have the tools
to modify the contents of the partition image after it is created. For
ext2/3/4 filesystems we have the debugfs tool and for msdos/vfat
filesystems we have the mcopy tool. So for any of these filesystems we
skip the modification of the fstab file in the rootfs directory (and
skip the associated copy unless it is otherwise necessary) and update
the contents of fstab directly in the partition image.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>