introduce function to recurse down to the lowest layer-2 interface
corresponding to a given interface.
also re-introduce some of the previously removed input validation plus
some more to protect against glob and path based exploits.

readme: fix typo, improve formatting

Our build process works similarly to the image builder and doesn't support
the generation of initramfs-based images.

Fixes #616

Fixes #614

Fixes #177

Convert site.conf to JSON during build

docs: add D-Link DIR-505 (A1) and Buffalo WZR-HP-G300NH (and remove WZR-HP-G300N, it is not actually supported)

This will allow us to use its content from other languages than Lua as
well.

docs: fix typo

The timeout was calculated incorrectly (in each iteration the span between
the start time and the current time was substracted from the timeout
again), and would often become negative, causing warnings in the kernel
log.

sse-multiplex likes to get the header as fast as possible.

The statistics data is now cached for 5s, no need to send an event every
second. The new interval is 3s, which is coprime with the cache timeout.

Allows simple handling of cached events.

docs: Improve documentation of fastd and autoupdater config in site.conf

Fixes #589

docs: releases/v2015.2: add ubiquiti airgateway