Use a single build for aarch64 qemu and gem5!

Document the minimal aarch64 boot.

Rename -c to -B for uniformity with -b.

Add help for ./run and ./build options.

common

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+set_common_vars() {
+  arch="$1"
+  root_dir="$(pwd)"
+  buildroot_dir="${root_dir}/buildroot"
+  arch_dir="$arch"
+  if "$gem5" && [ ! "$arch" = aarch64 ]; then
+    arch_dir="${arch}-gem5"
+  fi
+  out_dir="${buildroot_dir}/output.${arch_dir}~"
+  config_file="${out_dir}/.config"
+  images_dir="${out_dir}/images"
+}