image,populate_sdk_base: move 'func' flag setting for sdk command vars

Setting the 'func' flag on the commands variables ensures that they are parsed
as shell, and therefore that the referenced commands contents are included in
checksums. Doing this only in image.bbclass means that this is missing in
recipes that are not images, but which inherit populate_sdk or populate_sdk_base
directly, so move it to the latter.

[YOCTO #13998]

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit edc28907)
Signed-off-by: Steve Sakoman <steve@sakoman.com>