Dump device

Hello.
I'm trying to build FreeBSD-7.1, but have a problem - it panics after
"Trying to mount root from ufs:/dev/ar0s1a " and says "Fatal double
fault". Before sending bug report I want to look at core dump, but I
haven't found how  to set dumpdev variable in kernel (setting it in
loader doesn't work). Could you assist me? 

P.S. Setting dumpdev in rc.conf will not work, because system can't
mount root fs and read it.

-- 
Best regards, 
Alexander Pyhalov, 
system administrator of Computer Center of South Federal University