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.
Add help text; add configuration targets

This is step one in getting rid of the NERVES_BR_CONFIG variable that
seems to cause more confusion than it's worth.
Move to use unionfs for updating code on device

This addresses the problem of running relsync and messing up the
firmware on the device so that a reboot will fail.
Pull in am335x and fwtool updates
Update to latest buildroot and patch versions
Minor cleanup of the release images
Pull in relsync crash fix
Pass parameters to erlinit now
Clean up build by using BR2_EXTERNAL

This significantly reduces the number of patches that need to be
maintained to build Nerves and cleans up the main Makefile.
Switch kernel compression from XZ->gzip

Reduces boot time by 0.5 seconds. Small increase in image size.
Fix error when no files need to be stripped
Fix old references to br-configs
nerves-minimal: Add minimal nerves application

This has no external dependencies so it won't cause non-buildroot
initiated downloads when building the SDK. It also should run on any
platform that can run Erlang since it is basically a hello world app.
nerves-demo: fix URL
configs: use nerves-minimal app for sdk image
mmccopy: bump version to 1.0.0
erlang-relx: switch to prebuilt release

Erlware publishes pre-built versions of relx on Github, so use those
instead of building relx from scratch.
mmccopy: fix version spec

Thanks to ghitchens@gmail.com for catching my mistake.