sh: Fix up uImage target entry point.

This patch changes the uImage target so that it generates a wrapped
compressed vmlinux, rather than a wrapped zImage. The previous version
matched the ARM, this version matches the PPC. However I would question
how useful a self decompressing image is with a boot loader which does
decompression, so I think this is more useful. I also feel it matches
the descrition in the help text ("Compressed kernel image") better.

Signed-off-by: Thomas Betker <thomas.betker@5etech.eu>
Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>