configure: linux-user doesn't need neither fdt nor slirp

configure: linux-user doesn't need neither fdt nor slirp

if softmmu is not enabled, we disable by default fdt and
slirp as they are only used by -softmmu targets.

A side effect is the git submodules are not cloned
if they are not needed.

Clone and build can be forced with --enable-fdt and
--enable-slirp.

Signed-off-by: Laurent Vivier <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20190621130544[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>