Vagrantfile: Use VBox's linked clones.

This reduces the time and space required to spin up a new VM.

Vagrantfile

@@ -9,6 +9,7 @@ Vagrant.configure("2") do |config|
       machine.vm.network "private_network", ip: machine_config[:external_ip]
       config.vm.synced_folder ".", "/vagrant", disabled: true
       machine.vm.provider "virtualbox" do |vb|
+        vb.linked_clone = true
         vb.memory = 2048
         vb.customize [ "modifyvm", :id, "--uartmode1", "disconnected" ]
       end