[PATCH] add cuImage comment for 'make help'

It just take some time to figure out that there is no make cuImage
for the embedded powerpc stuff when you want to embed the
OF device tree into the target.

You have to use 'make zImage' instead.

The following patch adds a comment to the platform specific
'make help' section.

Signed-off-by: Clemens Koller <redacted>