cmake: modules: add support for board extensions

Board extensions are board fragments that may be present in any board
root folder, under `${BOARD_ROOT}/extensions/${BOARD}`. The board
extension directory may contain Kconfig fragments and/or devicetree
overlays.  Board extensions are automatically loaded and applied on top
of board files, before anything else. There is no guarantee on which
order extensions are applied, in case multiple exist.

Board extensions may be useful in the context of Zephyr modules, such as
`example-application`. In some situations, certain hardware description
or choices can not be added in the upstream Zephyr context, but they do
in a downstream project, where custom bindings or driver classes can
also be created. This feature may also be useful in development phases,
when the board skeleton lives upstream, but other board features are not
yet present.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>