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>
(From OE-Core rev: 2265d089a58e1f78f26d623ee667c420cb1c3bd4)

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

scripts/lib/wic/help.py

@@ -980,6 +980,12 @@ DESCRIPTION
                          copies. This option only has an effect with the rootfs
                          source plugin.
 
+         --change-directory: This option is specific to wic. It changes to the
+                             given directory before copying the files. This
+                             option is useful when we want to split a rootfs in
+                             multiple partitions and we want to keep the right
+                             permissions and usernames in all the partitions.
+
          --extra-space: This option is specific to wic. It adds extra
                         space after the space filled by the content
                         of the partition. The final size can go