mkosi: Optionally build a kernel image from mkosi.kernel/

mkosi: Optionally build a kernel image from mkosi.kernel/

If mkosi.kernel/ exists, the mkosi script will try to build a kernel
image from it. We use the architecture defconfig as a base and add
our own extra configuration on top.

We also add some extra tooling to the build image required to build
the kernel and include some documentation in HACKING.md on how to
use this new feature.

To avoid the kernel sources from being copied into the build or
final image (which we don't want because it takes a while), we put
the mkosi.kernel/ directory in .gitignore and use
"SourceFileTransfer=mount" so that the sources are still accessible
in the build image.