gem5: track build and run variants separately with -M and -N

Otherwise, checking out branches is too insane, as it does not
update the worktrees, even though the gem5/gem5 module was updated.

gem5: expose build types, document debug builds.

simultaneous runs: store stdout and stderr on a file to allow running
all from a single terminal on the background cleanly.

rootfs_overlay/gem5.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+m5 checkpoint
+script=/tmp/readfile
+m5 readfile > "$script"
+if [ -s "$script" ]; then
+  sh "$script"
+fi