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>

flavours/zephyr/conf-notes.txt

@@ -8,6 +8,7 @@ Supported machines (first is the default):
 - qemu-cortex-m3
 - 96b-nitrogen (96Boards Nitrogen)
 - 96b-avenger96 (96Boards Avenger96)
+- nrf52840dk-nrf52840 (Nordic Semiconductor nRF 52840 Development Kit)
 
 MACHINE variable can be set up in conf/local.conf file under build directory
 or via command line, e.g.:

flavours/zephyr/local.conf.sample

@@ -28,6 +28,7 @@
 #
 #MACHINE ?= "96b-nitrogen"
 #MACHINE ?= "96b-avenger96"
+#MACHINE ?= "nrf52840dk-nrf52840"
 #
 # This sets the default machine to be qemu-x86 if no other machine is selected:
 MACHINE ??= "qemu-x86"