kconfig: misc: Conditionally remove FLASH_LOAD options

This patch adds a condition on the FLASH_LOAD_OFFSET and FLASH_LOAD_SIZE
to make them conditionally excluded if HAS_DTS is enabled.  FLASH
partition information must be defined for all DTS platforms which
utilize flash.

For DTS enabled platforms, CONFIG_FLASH_LOAD_OFFSET and
CONFIG_FLASH_LOAD_SIZE will be generated from the flash information
defined in the DTS file.  The values used for these variables will be
determined by the zephyr,code-partition chosen node.  If no chosen
node is specified, the zephyr,flash chosen node will be used.

Signed-off-by: Andy Gross <andy.gross@linaro.org>