build: Run triggers and generate initramfs at build time

We already ship a kernel, so why not ship the initramfs as well?
This avoids clients doing it, which is more efficient for them.

Closes #7
build: Generate initramfs only if certain components change

There's no need to generate initramfs over and over because it wastes
several megabytes per build on the several and requires clients to
download the same initramfs every update.

Closes #11
Pick up overrides from $workdir/overrides/$name

This is much better than editing the manifest every time we want to
build from local sources instead of a git mirror.

Closes #12