linux: Add a load address option for building uImage on 3.7+ multi-platform kernels

[Peter: add quotes as suggested by Arnout]
Signed-off-by: Julien Boibessot <julien.boibessot@armadeus.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>