Add highly visible indicator that logging to SD is active.

[tonycanike]

Add a highly-visible indicator, observable from below, which shows that logging writes to the SD card are happening.

Static logging is a long-term operation, 2, 4, or even 8 hours. It is important to be able to verify that logging is actually happening after the Facet is setup. It is disheartening to find a setting was misconfigured or even the SD card wasn't properly inserted.

While the logging icon on the OLED display is very useful, the OLED display is often not visible.

1. It is very common to have a GNSS receiver up on a 2m pole. It's not uncommon for the pole to be on a mark that is projecting 0.2m above the ground. That puts the OLED display over 2.2m up in the air, well above eye level for most people.
2. The OLED display is easily washed out in bright sunshine.
3. The OLED display is slightly recessed, making it harder to read from low vertical angles. This is good as it protects the display, but the logging icon in in the lower part of the display.

Many commercial survey GNSS receivers use a bright blinking LED to indicate that logging is active. The LED is visible in sunshine and visible from a low vertical angle.