WIP stuff to get started on the pinebook pro.

## `u-boot`

Assuming `/dev/mmcblk0` is an SD card.

```
$ nix-build -A pkgs.uBootPinebookPro
$ lsblk /dev/mmcblk0 && sudo dd if=result/idbloader.img of=/dev/mmcblk0 bs=512 seek=64 oflag=direct,sync && sudo dd if=result/u-boot.itb of=/dev/mmcblk0 bs=512 seek=16384 oflag=direct,sync
```

The eMMC has to be zeroed (in the relevant sectors) or else the RK3399 will use
the eMMC as a boot device first.

Alternatively, this u-boot can be installed to the eMMC.

Installing to SPI has yet to be investigated.

## Image build

```
$ ./build.sh
$ lsblk /dev/mmcblk0 && sudo dd if=$(echo result/sd-image/*.img) of=/dev/mmcblk0 bs=8M oflag=direct status=progress
```

The `build.sh` script transmits parameters to `nix-build`, so e.g. `-j0` can
be used.

Once built, this image is self-sufficient, meaning that it should already be
booting, no need burn u-boot to it.

The required modules (and maybe a bit more) are present in stage-1 so the
display should start early enough in the boot process.

## Note about cross-compilation

This will automatically detect the need for cross-compiling or not.

When cross-compiled, all caveats apply. Here this mainly means that the kernel
will need to be re-compiled on the device on the first nixos-rebuild switch,
while most other packages can be fetched from the cache.