dts: stop generating CONFIG_FLASH_LOAD_{OFFSET,SIZE}

As a step to completing removing use of Kconfig defines in dts files we
need to change how we configure/set CONFIG_FLASH_LOAD_{OFFSET,SIZE}.

Previously we would set them based on how the chosen property
'zephyr,code-partition' was set to.  If 'zephyr,code-partition' wasn't
set we would default the values to 0.  We had some generic overlay logic
which would define 'zephyr,code-partition' based on
CONFIG_BOOTLOADER_MCUBOOT.

Going forward if the DTS has 'zephyr,code-partition' set we will
generate DT_CODE_PARTITION_OFFSET & DT_CODE_PARTITION_SIZE defines.  We
will leave it to Kconfig to set CONFIG_FLASH_LOAD_OFFSET &
CONFIG_FLASH_LOAD_SIZE.

We introduce a python script that allows Kconfig to extract data from
the DTS and thus we can utilize DT_CODE_PARTITION_OFFSET and
DT_CODE_PARTITION_SIZE values to set defaults for
CONFIG_FLASH_LOAD_OFFSET & CONFIG_FLASH_LOAD_SIZE.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>