implement variables in yaml

Variables in yaml would allow for conditional configuration, for example, system init type (systemd/sysv-rc/openrc)

```
$init == systemd:
  install:
    libpam-systemd
  script: | # enable ttySAC0
    systemctl enable getty@ttySAC0.service

$init == sysv-rc:
  script: |
    update-rc.d firstboot defaults
```

also $init would be exported to script sections as $DIBS_init