[OE-core] Permit to install files from an overlay in images recipes

[Geoffrey Levillain]

[PATCH 1/3] No need to force image licence on MIT
[PATCH 2/3] Add possibility to install overlays to image through a class
[PATCH 3/3] Do not remove fetching and install_overlay stamps

These patches introduce a capability to install overlay into image after
fetching them.

There is actually a need to add image-specific configurations files into
image recipes, and these patches are a way to standardize this process as
well as simplify it by re-enabling fetching when image_overlay is
inherited, and by providing a task that copy every files in directories
listed in OVERLAY_ROOT_DIRS into IMAGE_ROOTFS directory.

Regards,
Geoffrey Levillain