Enable git submodules so dtc/ is populated

Many machines do not have the device tree compiler installed or their
distro does not ship a suitable version.  Enable git submodules so we
pull in dtc from source and can build successfully.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Merge pull request #5 from stefanha/dtc-submodule-init

Enable git submodules so dtc/ is populated