Configure via boot files

**Just opening this pull request for discussion, rather than merging**

I've made a rough start on radiodan/project#2. Currently `/boot/wpa_supplicant.txt` and `/boot/hostname.txt` will be copied into the correct location if those files are newer.

Issues:
 - do these get copied too late in the process?
 - as well as editing `/etc/hostname` the name should be replaced in `/etc/hosts`

@pixelblend @chrislo