dfu: img_util: move flash defines to SoC

mcuboot_constraints.h had FLASH information related to the SoC that
should be maintained as part of the SoC and not in the subsystem. Also
fixed Makefiles to check for IMG_UTIL Kconfig and not MCUBOOT.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>