build: make baremetal parts more flexible and powerful

Document test-gdb on readme

test

@@ -1,4 +1,14 @@
 #!/usr/bin/env bash
 set -eu
-./bench-boot --size "${1:-1}"
+test_size=1
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --size)
+      test_size="$2"
+      shift 2
+      ;;
+  esac
+done
+./bench-boot --size "$test_size"
 ./test-modules
+./test-gdb