Add Installation of OS Images from USB

Hi,

for various use cases like
* users with a small SD card
* sharing one usb pendrive in a classroom
* (re)installing on an raspberry pi compute module
* ...?

it would be great to use os images stored on an USB storage device.
This PR mounts the first partition of an FAT formatted USB drive (`/dev/sda1`) if available and looks for additional os images in the `os` folder on it.

I've seen other people looking for something like this too: #164 and #213