dts: Remove mcuboot.overlay

As we want to remove dts dependency on Kconfig, we had a case based on
CONFIG_BOOTLOADER_MCUBOOT.  From a DTS point of view that was just
getting the chosen property 'zephyr,code-partition' set.  We can easily
move this to the actual dts files and remove the mcuboot.overlay.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>