zephyr: doc: Mention the DT flash description
zephyr: doc: Mention the DT flash description

In order to build for a particular Zephyr board, the Device Tree
definition for it in the Zephyr tree must include the flash description.

Additionally mention that a .hex version of the image is available,
useful for those flash tools that deal with Intel Hex files.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>

runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,tamban01/mcuboot