this requires a patch to uboot:
- uboot thinks the drive has a capacity of 0 (i.e. 'unknown'). unclear precisely why. could be noncompliant drive firmware, or a timeout somewhere.
and a patch to the rpi bootloader:
- in order to trampoline into the rpi-4 uboot.
and custom kernel modules in the initrd:
- in order to detect the USB hub (rpi fw).
additionally, i'm MANUALLY placing `bcm2711-rpi-400.dtb` into `/boot/nixos/..-linux-5.10.111-dtbs/broadcom`.
i'll want to do this automatically over time.
i hope to simplify much of this over time: this is just the first thing which works after a couple days of hacking at it.
notably, this avoids the problem where `co<tab>` can't auto-complete the trailing slash.
`cf<tab>` autocompletes to `cfg/` in the same number of keystrokes :-)
