Reorganize the way machine architectures are handled. Consolidate the

Reorganize the way machine architectures are handled.  Consolidate the
kernel configurations into a single generic directory.  Move machine-specific
Makefile's and loader scripts into the appropriate architecture directory.

Kernel and module builds also generally add sys/arch to the include path so
source files that include architecture-specific headers do not have to
be adjusted.

sys/<ARCH>            -> sys/arch/<ARCH>
sys/conf/*.<ARCH>     -> sys/arch/<ARCH>/conf/*.<ARCH>
sys/<ARCH>/conf/<KERNEL> -> sys/config/<KERNEL>