ARM: 7656/1: uImage: Error out on build of multiplatform without LOADADDR

On multiplatform kernels, $MACHINE will be empty so there will be no
default LOADADDR. Fail to build the uImage target unless one is provided by the
developer at build time.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>