The intent of the board revision mechanism is that users should be allowed to specify the revision of their actual board, which may not directly match a revision in the build system. This way, the maintainers can define new board revision files for major changes, and the build system will use the most recent revision's configuration until there is a new one.
The extension used to present this field as a drop-down of all defined revisions, but since this does not allow you to specify the exact revision of your board, it has been changed to a freetext field.