doc: zephyr: Refer to the BOOTLOADER_MCUBOOT Kconfig option
doc: zephyr: Refer to the BOOTLOADER_MCUBOOT Kconfig option

Since the sample application now uses the BOOTLOADER_MCUBOOT
configuration option, refer to it in the Zephyr documentation itself.

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

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