populate_sdk_ext.bbclass: Fix undefined variable error

The variable uninative_checksum is returned without being set, causing a
build error. Set it to None by default instead.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 69ead1f2)
Signed-off-by: Steve Sakoman <steve@sakoman.com>