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.

(From OE-Core rev: f01f0b8aed25af889f48fe1afff96feb3d9ed120)

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