boards: arm: add support for the Actinius Icarus SoM DK w/ helper lib

This adds support for the nRF9160-based Icarus SoM DK
(development kit) from Actinius and a common library for
board init code that is common to multiple boards
from Actinius.

Signed-off-by: Alex Tsamakos <alex@actinius.com>