Add indicator for "External Power Connected"

[tonycanike]

The green power indicator LED on the FACET only shows when the battery is charging due to external power being supplied.

It would be very useful to have an indicator to show when external power is connected, regardless of the state of the internal battery.

When setting up the base for a long work day, it is often necessary to connect external power, especially if using RTK radios, more especially if the radios are transmitting at more power than 100mW. It is important to be able to confirm that the external power is properly connected to ensure the work day will not be interrupted.

Currently, if the internal battery is fully charged, there is no indicator to confirm that the external power is connected properly. If the user thinks the external power is properly connected, but it actually is not, the base will shutdown midday due to the internal battery depleting, interrupting RTK work and static collection.