wic: Add --embed-rootfs argument

This option adds the content of a rootfs on a specific location on the
rootfs.

It is very useful for making a partition that contains the rootfs for a
host and a target Eg:

/ -> Roofs for the host
/export/ -> Rootfs for the target (which will netboot)

Although today we support making a partition for "/export" this might
not be compatible with some upgrade systems, or we might be limited by
the number of partitions.

With this patch we can use something like:

part / --source rootfs --embed-rootfs target-image /export --embed-rootfs target-image2 /export2

on the .wks file.

Signed-off-by: Ricardo Ribalda Delgado <ricardo@ribalda.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>