Feature: remotely flashing the board

Add support for flashing the board via the separate utility.
Server config should restrict this on a per-user basics:
For example:
```yaml
devices:
  - board: rb5-1
    all-other-properties
    flash:
      rootfs:
       -  someuser@host
       - anotheruser
      modem:
        -  admin@localhost
 ```