Enable dtc (libfdt) to applying overlays at runtime
Enable dtc (libfdt) to applying overlays at runtime

The Raspberry Pi device tree overlays can be applied at runtime using
the `dtoverlay` program that's already installed as part of the
`rpi-firmware` build. Unfortunately, it doesn't work unless libfdt.so is
around. This change adds it.

This feature is very interesting since it makes it possible to use Linux
kernel drivers without modifying fwup.conf and config.txt to load the
new device tree overlays! You can do things like turn GPIOs into LEDs
and buttons that are managed by the LED class drivers and Linux input
system.

nerves-project/nerves_system_rpi0,nerves-project/nerves_system_rpi0