standardize on extlinux bootloaders

[illegalprime]

There's a lot of unnecessary variety in boot loaders across boards, I think standardizing on uboot + extlinux is what's best for now.

• [x] get rid of unnecessary boot.loader.*.enable = false;
• [x] convert uEnv.txt boards into extlinux (this is the hardest one of the bunch)
• [x] use the config.system.build.installBootloader where appropriate instead of importing the builder directly like sd-image.nix does.
• [x] separate build-time and host-time extlinux builders
• [x] get rid of build-time binaries in builder
• [x] optionally refine the list of dtbs copied over to the /boot partition

machines to convert:

• [x] Odroid-C2
• [x] Raspberry Pi Zero (W)
• [x] Raspberry Pi 2
• [x] BeagleBone

this will make nixops and nixos-rebuild work better, but it may run into issues with mender... we'll see

[illegalprime]

done! also did raspberry pi 3