unified mkbootimage

I have converted prep and rs6000 to use the unified mkbootimage program located
in powerpc/stand/mkbootimage.  This should also work with bebox, but I cannot
test it.  It's also possible that it needs a few small tweaks.

Could someone please test it, and let me know if it works?  I can help with the
conversion of makefiles if needed.

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Tim Rightnour <root <at> garbled.net>
NetBSD: Free multi-architecture OS http://www.netbsd.org/
Genecys: Open Source 3D MMORPG: http://www.genecys.org/