cell: support multi-platform image

Paul,

This series of patches adds support to build multi-platform images with Cell BPA targets.

These patches apply to current powerpc.git, and should be considered for inclusion in 2.6.18

They have already seen some review on the
[email protected] ML.

-Geoff

I'll integrate them with my patches for 2.6.18 and send them on.

Thanks!

	Arnd <><