wic: Add --change-directory argument

This option allows to specify which part of a rootfs is going to be
included, the same way the -C argument on tar.

Thanks to this option we can make sure the permissions and usernames
on the target partition are respected, and also simplify the creation of
splitted partitons, not neeting to invoke external vars or using .wks.in
files. Eg:

part / --source rootfs --ondisk sda --fstype=ext4 --exclude-path=etc/   
part /etc --source rootfs --fstype=ext4 --change-directory=etc

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