sparc32: added U-Boot build target: uImage

This is only for LEON as u-boot for SPARC only supports LEON.

Signed-off-by: Daniel Hellstrom <daniel@gaisler.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>