cmake: board_check_revision() also accepts a list of valid revisions

To remove the need of empty config files, the `board_check_revision()`
now accepts a list of valid revisions.

As example:
```
board_check_revision(
  FORMAT MAJOR.MINOR.PATCH
  VALID_REVISIONS 0.1.0 0.5.0 0.10.0
)
```

The code is still compatible with the `<board>_<revision>.conf` changes
so that if different revisions of a board has Kconfig differences, then
there is no need to also specify the list of valid revisions.

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