Kernel crash before dumpon

[Frank Solensky]

I'm trying to get a dump off a machine with a 7.1-beta2 kernel that's
been crashing during the boot process, following the instructions on
http://www.freebsd.org/doc/en/books/developers-handbook/kerneldebug.html#KERNELDEBUG-OBTAIN
I've recompiled the kernel with "-g" but no vmcore file appears so I'm
assuming that the crash occurs before dumpon is executed.

The page includes the following suggestion on how I might be able to
proceed:
        Alternatively, the dump device can be hard-coded via the
        dump clause in the config(5) line of a kernel configuration
        file. This approach is deprecated and should be used only
        if a kernel is crashing before dumpon(8) can be executed.

I tried adding
        config dump "/dev/ad4s3b"
to the configuration file but that option appears to be no longer
supported: the config command gives an error message of:
        root/dump/swap specification obsolete

Is the paragraph above obsolete?  If so, what's the preferred way to
collect the dump?

Thanks