tools: Install tini

During development, we leave a container running and `exec` into it
to run multiple commands.  To keep the container running, we need a PID
1 process that never terminates, which we achieved previously with
Python.  This technically works, but ultimately leaves a lot of zombie
processes.  If we use a "true" init process for PID 1, it will clean
these up.

lib/tools.sh

@@ -16,4 +16,5 @@ sys-fs/dosfstools
 sys-fs/genimage
 sys-fs/mtools
 sys-fs/squashfs-tools
+tini
 EOF