wic: Add rootfs_dir argument to do_prepare_partition() method

wic: Add rootfs_dir argument to do_prepare_partition() method

The do_prepare_partition() method from RootfsPlugin class need
to know what will be the rootfs_dir. This makes sense when .wks
file has a partition set up like this:

  part /standby --source rootfs --rootfs-dir=<special rootfs> ...

then do_prepare_partition() will work with the correct rootfs.

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>