git: set --with-gitconfig=/etc/gitconfig for -native builds

Commit 6c2ae234 (kern-tools: depend on git-replacement-native)
broke our kernel builds. For saving space and time, we have a DL_DIR
shared between multiple users/buildbots, not all of which run with the
same uid (and with appropriate sticky bits set so that files
downloaded by one user become owned by a common group and are readable
by others). This works fine also for git sources because the docker
images we use all have a /etc/gitconfig with

  [safe]
    directory = *

But with the mentioned commit, the host's git is no longer used for
do_unpack (nor for do_fetch if re-building and sysroot has already
been populated by a previous build), causing spurious "fatal: detected
dubious ownership..." failures.

Currently, the path where the git-native binary searches for system
gitconfig is the sysroot from it was built, which obviously doesn't
contain a /etc/gitconfig. As for the nativesdk variant, respect the
host's /etc/gitconfig if present.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 572f511f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>