flavours: zephyr: add nRF 52840 DK support

Add relevant MACHINE entry in local.conf.sample of the Zephyr
flavour and metion it in configuration notes displayed
after sourcing the env with TEMPLATECONF.

Closes: OSTC/planning/core-os#168

Signed-off-by: Wojciech Zmuda <wojciech.zmuda@huawei.com>