contributing-device-support.md: Updates to board support instructions
contributing-device-support.md: Updates to board support instructions

Change-type: patch
Signed-off-by: Florin Sarbu <3a837a37a2ac9072a088641ec330e0e5d5b86f4e@balena.io>

resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin