Add special case code for the rPC44. The DMA buffers on this machine

Add special case code for the rPC44.  The DMA buffers on this machine
are not cache coherent, so for the moment use uncached addresses for
all buffer access.  In the future, more specific cache flushing should
be done.

rPC44 now boots into multi user off SCSI disk or the network, builds
its own kernel and seems to be making its way toward finishing a make
build.

Correct -Wall complaint when BTDEBUG defined while I'm here.