qemu: Fix slirp determinism issue

Add a PACKAGECONFIG option for slirp, defaulting to internal. This avoids
the presence of libslirp on the host causing qemu to link against that
instead breaking reproducibility and usability of the binary on hosts
where the library isn't present.

We need to add it to PACKAGECONFIG by default since users do expect slirp
to be enabled in the wider community.

Note: qemu version 4.2.0 doesn't support an "internal" option for
enable-slirp, so use "git" instead which uses the same configure
code path, avoids host libslirp contamination and forces use of the
qemu internal slirp implementation.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 5a9a6413)
Signed-off-by: Steve Sakoman <steve@sakoman.com>