mainboard sub-type support

[Ronald G. Minnich]

We have a mainboard called the p4dpe-g2. It seems to be almost identical 
to the p4dpe. 

I was faced with several options: 
- have a directory named p4dpe and a directory p4dpe-g2, with
  almost identical files

- Have a directory for the p4dpe, with #ifdef G2 in many files

- an interesting trick that works: 
  have a directory p4dpe, and a directory p4dpe/g2
  Populate p4dpe/g2 with the files: 
	cmos.layout, Config, mainboard.c, irq_tables.c, and mptable.c
	in other words, the files that will change with mainboard revs
	anyway

This last option works. The only change to the config file to build a -g2
motherboard is to change the line:
mainboard supermicro/p4dpe
to
mainboard supermicro/p4dpe/g2


Now this is kind of interesting. For a vendor like supermicro that spins 
lots of boards that are very similar, we can take this further: have
supermicro/mainboard/p4d/pr
for the p4dpr
and 
supermicro/mainboard/p4d/pe
for the p4dpe. These are similar boards, lots of shared code, and this 
structure shows that. 

Comments? is this good, bad, or indifferent?

ron