Build the Linux kernel independently from Buildroot

This will allow for other types of root filesystems that don't rely on Buildroot
to be added and used in the future.

Propagate --verbose on all build scripts to see full GCC commands.

build-all: allow for neat subsets

also 9p share rootfs_overlay. TODO document.

test

@@ -1,4 +1,4 @@
 #!/usr/bin/env bash
 set -eu
-./bench-boot -t "${1:-1}"
+./bench-boot --size "${1:-1}"
 ./test-kernel-modules