zephyr: boards: Add disco_l475_iot1 config file

The STM32L475 needs BOOT_MAX_IMG_SECTORS equal 256 to work properly.
This add a config file to change default value from 128 to 256 once
there isn't a proper way to handle using Device Tree.

fixes zephyrproject-rtos/zephyr/issues/24243

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>