build: unify QEMU and gem5 kernel configs! The last one!

Split optional and required kernel configs into separate files.

readme: describe how our kernel configs are generated from Buildroot

readme: document QEMU `-no-reboot` and speculate gem5 `panic_on_panic`
in preparation for some more automated bisection with ktest.

common

@@ -24,9 +24,6 @@ set_common_vars() {
   gem5="${2:-false}"
   buildroot_dir="${root_dir}/buildroot"
   arch_dir="$arch"
-  if "$gem5" && [ "$arch" = x86_64 ]; then
-    arch_dir="${arch}-gem5"
-  fi
   out_arch_dir="${out_dir}/${arch_dir}"
   buildroot_out_dir="${out_arch_dir}/buildroot"
   build_dir="${buildroot_out_dir}/build"