slightly updated recipe to config/build u-boot for the BBB

i've posted this before but i updated slightly my wiki page for how
to build u-boot for the BBB from scratch:

  mostly, i just added the comments graciously posted on this list by
tom rini in this section:

  as i mentioned once upon a time, the rationale for that page is for
a training class in which students will have to build the individual
components for a bootable system bit-by-bit, so that page represents
how i'm going to ask them to build their MLO and u-boot.img
components. and i suspect part of the lab will involve going through
the patches and deciding which are necessary and which aren't -- just
a general discussion to see how the generic u-boot sometimes needs to
be tweaked for a particular board.

  i note tom rini's reference to the timesys u-boot tree, so i might
take a look at that as well (is it publicly available or do i need a
linuxlink subscription?).

  in any event, if anyone has suggestions for cleaning up/improving
that page, let me know; i'll be happy to add them.