When debugging a build (or maintaining an existing packer file), teach `packer build -debug` how to step through individual `boot_command`s in order to triage the packer file.
```
==> vmware-iso: Typing the boot command over VNC...
==> vmware-iso: Pausing after run of step 'boot_command[0]: <enter><wait>'. Press enter to continue.
==> vmware-iso: Pausing after run of step 'boot_command[1]: <enter><wait>'. Press enter to continue.
==> vmware-iso: Pausing after run of step 'boot_command[2]: freebsd-vagrant<enter><wait>'. Press enter to continue.
==> vmware-iso: Pausing after run of step 'boot_command[3]: <down><spacebar>'. Press enter to continue. ^C
```