DT Overlay and extlinux.conf support

Please see https://github.com/openbouffalo/buildroot_bouffalo/wiki/U-Boot-Bootflow for details how the uboot/extlinux.conf files work for DT Overlays

What's Changed

• DTB Overlay Support and extlinux.conf by @Fishwaldo in #43

Full Changelog: v1.0.0...v1.0.1