ARM: nomadik: convert to multiplatform

This converts the Nomadik to run in multiplatform mode, including
the defconfig change. After this the "uImage" target in the kernel
tree will no longer work, but we do not care about this. Instead
we generate the uImage from the zImage using mkimage or update
the bootloader to accept bootz. Some minor updates to the defconfig
are done as part of this.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>