Integrate Raspberry Pi 4 EEPROM updates into the netboot procedure

[paxswill]

There's some sort of mechanism for the RPi4 to update it's firmware the same say it netboots.

[paxswill]

Turns out this is easier than I thought:

• Extract the current bootloader config. This can probably be skipped if I'm going to set all of the nodes to use the same config.
• Patch the firmware file with the desired config. The default is to just copy the existing config. That entire script has parts of what's needed for the binary patching.
• Generate a signature file for the patched firmware file
• Stage both the firmware and signature (named pieeprom.upd and pieeprom.sig respectively) in the TFTP root.

On startup, the Pi checks for the signature file during startup. If the timestamp in the signature is newer than the timestamp in the current firmware, the Pi loads the actual update file, writes it, and continues on it's merry way (I think that last part is true. The Pi might to a reboot, then boot.).

The hard part of this is that the scripts from raspberrypi/rpi-eeprom assumer in a number of ways that they're running on Raspbian:

• They assume a Raspberry Pi Foundation device tree has been used (they attempt to load the current config from /sys/firmware/devicetree/base/aliases/blconfig).
• vcgencmd is used as a fallback for accessing the current bootloader config.

Solving all of this is outside of the scope of this project, and could probably be coordinated with Debian/Ubuntu better:

• Ubuntu has packaged raspberrypi/userland (which provides the libraspberrypi0 and libraspberrypi-bin packages, which provide vcgencmd).
• Debian isn't packaging the RPi 4 firmware yet. I should really write up that proposal to split raspi-firmware into the firmware files and that install scripts, and then propose adding a new package for the RPi4/RPi400/CM4 firmware, and another package that provides the update scripts for them.
• Check to see if flash-kernel can update firmware too.