scripts: dtc: Fetch fdtoverlay.c from external DTC project

We will start building overlays for platforms soon in the kernel and
would need fdtoverlay tool going forward. Lets start fetching it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/28f66f70602225bb6aeb58e924c20bde9d864327.1611904394.git.viresh.kumar@linaro.org