This adds a `CONFIG_UROOT=y` option to allow the busybox
runtime to be replaced with the go u-root runtime.
You must have go 1.9 or newer for it to work.
It has been tested on the OCP winterfell and qemu nodes,
and it can be specified on the build command line as well.
Nothing from `heads/initrd` or any of the tools will be
linked into the cpio file. Only the kernel modules and the
go shell will be included.
Move board configuration into `boards/` instead of `config/`
Fix mistake in building kernel module tree before kernel was done.
Allow per-board initrd builds (#278)
Allow per-board configurations for things (#304)
