[coreboot] failover only for s2895

[Myles Watson]

I had always misunderstood failover.  I thought it was the last resort if
normal & fallback didn't work.  It actually is just a minimal image that
chooses whether to run normal or fallback.

This patch makes that clear for the s2895 by separating the files.  I like
it much better.  I'm eventually planning to do the same for all of the AMD
Tyan boards.  For the patch to apply, you have to svn cp cache_as_ram_auto.c
to failover.c

The only problem I had was where to put the definition of post_code.  Since
it is trivial, I made it a #define.  I tried to put it in other places, but
it affected the PPC targets and others because we define post_code in other
places.

Boot tested.

Signed-off-by: Myles Watson

Thanks,
Myles