cmake: BOARD_REVISION_CONFIG setting for Kconfig revision fragments

Fixes: #53696

Create a BOARD_REVISION_CONFIG setting to be consistent with the
BOARD_DEFCONFIG setting.

This allows systems which re-uses the Kconfig module to overrule the
file to be used as BOARD_REVISION_CONFIG in same way as is done for
the BOARD_DEFCONFIG file.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>