travis.yml: Use the libfdt from the distro instead of the submodule

travis.yml: Use the libfdt from the distro instead of the submodule

No need to compile-test third party submodules over and over again if
we can simply use the pre-build library from the distribution instead.

By also adding --enable-fdt=system to the configure options, we can
also avoid to check out the "dtc" submodule here.

Message-Id: <20230120075330.2076773-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>