bootp: remove the USE_BFUNCS knob

[kevans]

Summary

We'd likely be better served by converting these to the equivalent mem*
calls, but just kill the knob for now. The b* macros being defined get
in the way of _FORTIFY_SOURCE.

[imp]

When I see

/* Yes, memcpy is OK here (no overlapped copies). */

I cringe at the certainty of that assertion.
I applaud its departure.