sh: Fix compression method when making uImage.

When uImage is made by using 'make uImage', zImage is used.
If zImage is used, the compression method need not be set.
However, it is set for "gzip" for a compression method.
I corrected to set "none".

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>