nrf/Makefile: Add support for flashing with nrfutil.
nrf/Makefile: Add support for flashing with nrfutil.

An additional Makefile parameter NRFUTIL_PORT can be set in order
to define the serial port to used for the DFU (Default: /dev/ttyACM0).

The "nrfutil" that is used as flasher towards OpenBootloader is
available for installation through Python "pip".

In case of SD=s140, SoftDevice ID 0xB6 is passed to nrfutil's package
generation which corresponds to SoftDevice s140 v6.1.1.

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython