scripts: dtc: Build fdtoverlay tool

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

The fdtoverlay program applies one or more overlay dtb blobs to a base
dtb blob. The kernel build system would later use fdtoverlay to generate
the overlaid blobs based on platform specific configurations.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/4a201dea3ba11a00cab7e936dfc1140dac1a1ae3.1611904394.git.viresh.kumar@linaro.org