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.

rootfs-post-build-script

@@ -4,8 +4,9 @@
 # which gets called by the default target.
 target_dir="$1"
 mkdir -p \
-  "${target_dir}/mnt/9p" \
-  "${target_dir}/mnt/out" \
+  "${target_dir}/mnt/9p/data" \
+  "${target_dir}/mnt/9p/out" \
+  "${target_dir}/mnt/9p/rootfs_overlay" \
 ;
 # Maybe there is a cleaner way to get rid of those files,
 # like disabling some Buildroot packages, but no patience.