Implement network booting support via PXE and NFS

[samip5]

I think our initramfs should support PXE/NFS booting.

[jonesmz]

Wouldn't this be better left to the rpi's firmware and/ or uefi add-on in /boot?

The RPI itself can netboot iwith. I think, tftp, if you toggle the firmware setting, and then the uefi stuff from https://github.com/pftf/RPi4 rounds out the netbooy story

[samip5]

initramfs still needs support for NFS mounting options if I were to boot it via PXE because those are things that need to parsed from cmdline.

[jonesmz]

Ah, that's fair.