meson.build: Don't use internal libfdt if the user requested the system libfdt

meson.build: Don't use internal libfdt if the user requested the system libfdt

If the users ran configure with --enable-libfdt=system, they likely did
that on purpose. We should not silently fall back to the internal libfdt
if the system libfdt is not usable, but report the problem with a proper
message instead.

Message-Id: <20210827120901.150276-3-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>