brcmutil: add helper function to format board revision

The board revision that is available in hardware can be translated
so it matches the labelling on the board. This is accomplished by
this helper function.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>