scripts: ci: check_compliance: add BOARD_REVISION in Kconfig.board.v2

This is required if boards make use of such definition in their Kconfig
files. In Kconfig.board.v2, only `boards/Kconfig.v2` is loaded, but
BOARD_REVISION is part of `boards/Kconfig`, which can't be loaded in this
context.

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