update 83815 boot driver to recognise SiS controllers
and set their MAC adddresses (from ea= currently,
since it's a nuisance to dig it out of ROM).
also update load.c to make the above work by
probing ethers only after reading plan9.ini.

with these changes, I can boot through an sis controller,
giving ea= in plan9.ini.

--- added 1 Mar 2006 02:57 by geoff
Drat.  The change to load.c breaks 9pxeload.
I guess this needs more work.