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-boot

@@ -2,21 +2,18 @@
 set -eu
 root_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null && pwd)"
 test_size=1
-OPTIND=1
-while getopts t: OPT; do
-  case "$OPT" in
-    t)
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --size)
       # 1: a few seconds and important
       # 2: < 5 minutes and important or a few seconds and not too important
       # 3: all
-      test_size="$OPTARG"
-      ;;
-    ?)
-      exit 2
+      test_size="$2"
+      shift 2
       ;;
   esac
 done
-shift "$(($OPTIND - 1))"
+
 if [ $# -gt 1 ]; then
   extra_args=" $*"
 else