mkosi: Rework kconfig handling

mkosi: Rework kconfig handling

Instead of relying on the arch specific kconfigs that enable all
kinds of options that are irrelevant for our use case, let's beef
up our own config a little so that we don't need to rely on the
arch specific configs anymore.

Also, instead of using scripts/config to enable options in the
mkosi build script, let's add a new kernel.config file that includes
all the configs we want enabled/disabled.

Finally, let's make sure we can properly run the bpf selftests by
making sure we enable all the right configs required to run the bpf
selftests successfully. We use the scripts/kconfig/merge_config.sh
script to merge the three different config files together.