linux: Add support to specify special Kernel Image make target

This is useful for ARCHes like PowerPC that have a whole lot of
special targets for each different board.  The kernel image target
tells make which OpenFirmware machine description file is combined into
the kernel binary.

[Peter: fix long lines]
Signed-Off-By: Martin Hicks <mort@bork.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>