convert this Makefile to be more similar to others in distrib/**.
XXX: this still needs work to make this more non-netbsd-cross friendly.
e.g, use ${PAX} and compress instead of assuming that tar supports `Z', etc.
installation kernel into the BtNetBSD thus helping the installation
process on Acorn32 tremendously.
New users now only have to download the BtNetBSD.tar.Z image, unpack it
with the suplied !SparkPlug and run the installation kernel as they
normally would expect to be able to under RiscOS.
