wic: extra partition plugin

wic: extra partition plugin

The extra_partition plugin allows populating an extra partition with
files listed in the new IMAGE_EXTRA_PARTITION_FILES variable. The
implementation is similar to the bootimg_partition plugin.

This plugin provides an easy way to install files that are not part of
the rootfs, from the deploy directory.

Signed-off-by: Pierre-Loup GOSSE <pierre-loup.gosse@smile.fr>
Reviewed-by: Yoann CONGAL <yoann.congal@smile.fr>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>