doc: Document support for .hex files in imgtool
doc: Document support for .hex files in imgtool

Specify in the documentation that .hex files are supported by imgtool
when signing.

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

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