boards: rename adafruit_feather

Adafruit feather is a generic name that could match many
boards. Renamed to reflect actual board name:
- Adafruit Feather nrf52840 Express

There are two boards based on the nrf52840, so keeping directory
generic to support additional board defs with mostly shared dts.

Signed-off-by: Jacob Winther <jacob@9.nz>