dfu: boot: mcuboot: Convert FLASH_WRITE_BLOCK_SIZE to DT_
dfu: boot: mcuboot: Convert FLASH_WRITE_BLOCK_SIZE to DT_

Use DT_FLASH_WRITE_BLOCK_SIZE prefixed defined instead of
FLASH_WRITE_BLOCK_SIZE as the non-DT version is deprecated.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,ldts/zephyr,explora26/zephyr,galak/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,finikorg/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,finikorg/zephyr,explora26/zephyr