Fix license notices for OE-Core

* Fix up LICENSE:
  - Replace reference to Poky with OpenEmbedded
  - Remove reference to bitbake and meta-extras that aren't part of this
    repository
  - Clarify license statements
* Add a COPYING.GPLv2 file in meta/ for the GPLv2 parts
* Add a COPYING.MIT file to meta-skeleton consistent with the other
  meta-* directories

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>