Add base image preparation script and all its dependencies.

Currently just good enough to:
- Download the image
- Verify its checksum
- Write it to the SD card
- Inject files in the SD card so that the device:
  - Has networking
  - Fetches basic packages
  - Has proper date and somewhat okay timekeeping given the lack of RTC (through tlsdate and writing the time to disk)
  - Tor as DNS resolver
  - Has SSH set up with proper GPG key for access