nativesdk: Convert to use classextend.py

nativesdk: Convert to use classextend.py

This patch converts the nativesdk class itself from operating as a suffix
to a prefix (see the proceeding patch for the related changes outside this
class).

The big benefit here is that we can reuse the generic class extension code.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>