Extract kernels properly

[vanrein]

Currently, they are installed in rootfs, and instead the boot directory might be placed next to it. But that is still a bit weird. Find a good way and implement it.

Note that multiple kernels may be provided from a single request, so a simple solution is probably the boot directory next to rootfs. But that is specific to Linux too :'-(

[vanrein]

One approach might be to install the kernel, but build cpio archives over less than the full rootfs. Prefixes (that is, CPIO archives without a trailer) might be created holding the kernel, and/or the modules for a kernel. The kernel is used manually, so an extraction action might be usable.

The "mkhere" environment might give hints about extraction. In fact, the /mkhere/oskernel.sh list and /mkhere/oskernelmodules.sh list commands do that already.