powerpc/boot: Only build board support files when required.

powerpc/boot: Only build board support files when required.

Currently we build all board files regardless of the final zImage
target.  This is sub-optimal (in terms on compilation) and leads to
problems in one platform needlessly causing failures for other
platforms.

Use the Kconfig variables to selectively construct this board files to
build.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>