nrf/readme: Update link to nrfjprog download.
nrf/readme: Update link to nrfjprog download.

After new layout of nordicsemi.com the direct links to
command line tools (nrfjprog) has changed to become dynamic.
This patch removes the old direct links to each specific OS
variant and is replaced with one single link to the download
landing page instead.

pozetroninc/micropython,bvernoux/micropython,pozetroninc/micropython,pfalcon/micropython,MrSurly/micropython,selste/micropython,selste/micropython,MrSurly/micropython,pozetroninc/micropython,henriknelson/micropython,pfalcon/micropython,adafruit/circuitpython,pramasoul/micropython,pramasoul/micropython,bvernoux/micropython,pozetroninc/micropython,tobbad/micropython,pramasoul/micropython,kerneltask/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,MrSurly/micropython,pramasoul/micropython,pramasoul/micropython,kerneltask/micropython,henriknelson/micropython,kerneltask/micropython,kerneltask/micropython,trezor/micropython,tobbad/micropython,adafruit/circuitpython,pfalcon/micropython,pfalcon/micropython,MrSurly/micropython,selste/micropython,henriknelson/micropython,trezor/micropython,pozetroninc/micropython,bvernoux/micropython,trezor/micropython,pfalcon/micropython,bvernoux/micropython,tobbad/micropython,henriknelson/micropython,trezor/micropython,tobbad/micropython,tobbad/micropython,MrSurly/micropython,kerneltask/micropython,adafruit/circuitpython,bvernoux/micropython,trezor/micropython,henriknelson/micropython,selste/micropython,selste/micropython