cleaned up wiki page -- kernel modules you need for BBB

aside: today is a wiki cleanup day so i'll be updating a number of
pages, many of them related to the BBB. my top-level BBB page is here:

in case you're interested, and i just tidied up this one:

which describes the kernel functionality you need to talk to your BBB
right out of the box.

  i might add something about "make localmodconfig" to that page
later, if you're not familiar with that cool kernel config target.