Changelog:
===========
* #419: Declared "Distribution" as an abstract class, enforcing
definition of abstract methods in instantiated subclasses. It's no
longer possible to instantiate a "Distribution" or any subclasses
unless they define the abstract methods.
(From OE-Core rev: 864fd15f05f3e9e795d272239be30db4a19c9ceb)
Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
