boot: zephyr: add MCUBOOT config
boot: zephyr: add MCUBOOT config

Add a hidden MCUBOOT config entry to mark a project as MCUBOOT.
It is useful when other parts of the system need to be aware
that they are, in fact, the bootloader.

Signed-off-by: Emanuele Di Santo <58b3ebaa556fff9ee88aab1044a35f0af0dd7061@nordicsemi.no>

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