buildtools-tarball: handle missing host pkg-config gracefully

buildtools-tarball: handle missing host pkg-config gracefully

The host might not have pkg-config installed, which is OK when using the
buildtools-extended-tarball, since that contains pkg-config. So the command in
the generated environment-setup-* script for setting HOST_PKG_PATH is expected
to fail in some cases without this being an error.

The generated environment-setup-* script might be sourced in a shell with
activated `errexit` option. Prevent the expected failure of this command from
being treated as an error.

The only usage of the HOST_PKG_PATH variable is in this script and already
handles the case of the variable being empty. The variable is not exposed to
anywhere outside this script so the effects of this change are limited to this
script.

Signed-off-by: Daniel Wagenknecht <dwagenknecht@emlix.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>