mass-update: improve flashing of low_flash and low_mem devices

[Noki]

Devices with low_flash / low_mem tend to run out of memory. We should therefore improve the mass-update script in the following way:

• reboot device before flashing
• stop services to free memory before flashing
  • /etc/init.d/collectd stop; /etc/init.d/luci_statistics stop; /etc/init.d/sysntpd stop; /etc/init.d/urngd stop; /etc/init.d/rpcd stop; /etc/init.d/naywatch stop
• wait a couple of seconds before flashing for everything to settle

After flashing most of these devices loose the overlay and therefore require a new password to be set we should therefore:

• check all flashed devices for a set password
• list devices that require manually setting a password