sanitycheck: add notes key to schema

Prior to addition of schema validation running sanitycheck
--generate-hw-map would preserve the value of any field in an existing
map that it didn't update.  These fields now cause a parsing error.
Add notes as an optional key where information relevant to the board
can be added to provide useful context (such as which serial port
corresponds to the console, or why a non-standard runner is required).

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>