uninative: Handle PREMIRRORS generically

Currently uninative handles SOURCE_MIRROR_URL but not generic PREMIRRORS.
It can handle this better, attempt to iterate PREMIRRORS entries.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6426c952)
Signed-off-by: Steve Sakoman <steve@sakoman.com>