samples: move board overlay files into boards directory

Application board.overlay files tend to be paired with
boards/board.conf files that extend the functionality of a board.
Move the overlay files to the same location as the config files that
they work with.

A few overlay files that are paired with a prj_board.conf file in the
application root directory are left in place.

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