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.

bench-all

@@ -117,7 +117,7 @@ fi
 if "$bench_linux_boot"; then
   cd "${root_dir}"
   "${root_dir}/build-all"
-  "${root_dir}/bench-boot" -t 3
+  "${root_dir}/bench-boot" --size 3
   cp "$(${root_dir}/getvar bench_boot)" "$new_dir"
 fi