Keep RSA/DSA key between reflash

The key should be stored somewhere where we can keep it between reflashing. Or at least it should be export/import during the flashing procudure in order to avoid generating it at each time...

Pros:

• This reduce the time to flash
• This avoid breaking the ssh if someone unplug the device during the generating of SSH key (~40s)
• This avoid security warning and asking to accept new key if the SSH was previously used.