Add network boot support

[jaxxzer]

https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/net_tutorial.md

[jaxxzer]

I think, this could hypothetically allow for the entire software for the system (ardusub, companion, qgc) to be bundled into a complete package, residing on the client computer. Something like this could potentially be an alternative to the use of a bootloader (or complementary) in #124.