Rewrite build script to support build environments, separate config profiles

[jheretic]

The current build script is pretty barebones. It just runs through a number of operations that generates an OpenWRT build tree and loads our configs. We should have a script that takes advantage of OpenWRT's build environment support, and that allows for different configuration profiles. This will make it easier to support custom configurations for specific deployments, for debugging, etc. The script should ideally support both an interactive and non-interactive mode.

[jheretic]

The https://github.com/technosopher/OpenWRT-fastbuild project already includes almost all of this functionality; @technosopher and @hawkinswnaf just need to adapt it to also serve as a replacement for the current in-tree build script.

[technosopher]

Starting work on this now.

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[jheretic]

I am moving this to 2014. This should be a high priority in January, however.

[jheretic]

Completed as per 1d549fd.