populate_sdk_ext: Do not assume local.conf will always exist

Some distros may not have local.conf, and use auto.conf or site.conf
alone to describe distro config metadata, therefore make code robust
to not fail eSDK builds for such setups

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6379f951)
Signed-off-by: Steve Sakoman <steve@sakoman.com>