Wifi support: add configuration and notes

This is a work-in-progress since Erlang modules are not available yet to
take advantage of it. Notes are in the README.md file for detecting
access points and then using the wpa_supplicant to connect to one.
Fix busybox config rename in all configurations
Pull in latest buildroot + am335x-pru-package

This also pulls in Erlang R16B03-1.