ARM: 7021/1: Check for multiple load addresses before building a uImage

uImages need a load address specified. This makes them
incompatible with multiple zreladdrs. Catch this error
before building an uImage so that we do not end up with
broken uImages. The load address can still be specified
with LOADADDR= on the command line.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>