Improved pilight uuid generation

Previously, pilight generated the UUID by using the mac address of
(one of) the wired network adapters. Almost all mainboards have at
least of one such network adapters present, except the Raspberry
Pi model A. So we need a different method to generate a fixed
unique ID for all pilight instances.

This new code will use the Rasbperry Pi's serial number retrieved
from /proc/cpuinfo so we can also generate a UUID on model A
boards. It will also dynamically retrieve network adapters and use
one of them to build a UUID based on its mac address.