Minimal custom userspace

[neuschaefer]

I'd like to build something like this with buildroot:

• [x] Simple httpd serving a simple page with usage instructions
• [x] Interaction mainly via ssh
  • [ ] user bmc for day-to-day BMC tasks (power on/off, SoL, etc.)
  • [ ] user sol to enable ssh sol@1.2.3.4 as a quick way to start a serial-over-LAN session
  • [x] persistent storage for SSH pubkeys and other bits of configuration
• [x] VNC (connected to video capture and USB input emulation) listening on localhost; forwarding via ssh -L ... → https://github.com/neuschaefer/wpcm450/issues/4
• [x] some sort of virtual media service, qemu-nbd → https://github.com/neuschaefer/wpcm450/issues/6
• [x] version information
• [ ] update mechanism
• [x] run shellcheck on all shell scripts