licence_image: Add lic-pkgs IMAGE_FEATURE

Installing license packages is similar to installing -dev or -dbg
packages, so let's invent a "lic-pkgs" IMAGE_FEATURE that does so and
document it in core-image.bbclass.

This image feature only works if LICENSE_CREATE_PACKAGE is set, so
refuse to generate an image if the lic-pkgs feature is enabled without
LICENSE_CREATE_PACKAGE.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>