boards: adafruit feather nrf52840 update UF2

Add support for flashing the Adafruit Feather nrf52840 Express
using UF2 over USB.

Also standardise on using the UF2 variant, so add a non UF2 variant
of the Sense board, so it can be flashed using the SWD headers.

Created flash dtsi files for both SWD and UF2. These are actually identical
for many other boards and should probably be moved to common in future.

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