ixp4xx-kernel: create generic and machine specific images/packages in 2.6.15

ixp4xx-kernel: create generic and machine specific images/packages in 2.6.15
 - the generic ixp4xx-kernel package now creates the same generic kernel
   regardless of how MACHINE is set, it then appends shims to the kernel
   to obtain machine specific kernels and produces packages of the results.
   Still TBD is the command line fixup.
   The generic command line is the nslu2 one, this is a temporary hack
   to keep the NSLU2 build working.