config(1): Fix build of old tree (bin/49389)

Define the default, empty "build_kernel" target, so that old source trees,
whose sys/conf/Makefile.kern.inc don't have "build_kernel" .USE target, can
be built.

(When "build_kernel" is defined, the target is overriden, as far as
sys/conf/Makefile.kern.inc is included later than the "netbsd: ..." definition
in sys/arch/*/conf/Makefile.*.)

This should address PR bin/49389.
Load the right xfer address into cb[i].xferdma. Fixes disk detection on
my hp715/50.

Hi Taylor.