build: make baremetal parts more flexible and powerful

Document test-gdb on readme

build-test

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+set -eu
+test_size=1
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --size)
+      test_size="$2"
+      shift 2
+      ;;
+  esac
+done
+./build-bench-boot --size "$test_size"
+./build-test-gdb