uboot:board: add generic platform and baseboard revision detection

This commit implements generic platform and baseboard revision detection
for SBC and SOM systems. It currently supports PFID based baseboard
detection.

- baseboard.h: common interface to query baseboard type and revision. It
  defines the board types, detects whether PFIDs are used or some other
  system and forwards the calls to the proper implementation.
- baseboard_pfids.h: type and revision detection using PFIDs
- baseboard_util.h: utility functions for reading GPIO values

Some additional functions were made available in kuk_boards.h to
implement the changes.