warp7: Make CONFIG_SYS_FDT_ADDR a define

warp7: Make CONFIG_SYS_FDT_ADDR a define

In order to sign images with the IMX code-signing-tool (CST) we need to
know the load address of a given image. The best way to derive this load
address is to make it into a define - so that u-boot.cfg contains the
address - which we can then parse when generating the IMX CST headers.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>