defined in vsprintf.h.  (This may change, since vsprintf.h is a
non-standard name, but for now it's the one to use.)

There should be no need to include vsprintf.h just for DBG() statements,
since include/compiler.h forces it in for a debug build anyway.

stack or INT 13 emulator etc.

period before updating to the new driver API (which can cope with
having TX packets in progress).

(This actually matters once you try using TCP, because you *will*
receive full-sized frames.)

Update rtl8139 driver to instantiate an SPI interface with a three-wire
device attached.

(since SPI bit-bashing will require different delay semantics).