linux/brcm47xx : Belkin F7Dx30x board detection

boardnum is unstable, so allow using NULL to make it unchecked.

A method for differentiating F7D3301 from F7D3302 using nvram is
unknown at this point. Reading the first 32bits of magic from the
"linux" mtd partition would do the trick, but these 2 are similar
enough that we don't need separate led/button definitions. Not knowing
which one you have is only a usability issue for upgrading the device:
they expect different magic numbers for their trx images.

Signed-off-by: Cody P Schafer <devel@codyps.com>

SVN-Revision: 38767