overlaycheck: Rewrite as a Python script

Reinvent overlaycheck as a Python utility that, for the most part,
replaces the pairing of ovmerge as a preprocessor and separate passes
for each test with the internalisation of the DT as a data structure
on which the various checkers work. The end result has been a new bug
found (apds9660 wasn't referencing its pinctrl node) and a two thirds
reduction in run time.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

Author: pelwell