dumptrees: pass outputdir as first argument

Instead of outputing files to current directory, allow to specificy an
output directory. This helps with meson build system out-of-tree support.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20191009102025.10179-4-marcandre.lureau@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>