native SOC: Add option to make a MCU to boot on its own

As a development helper, add a kconfig option to
automatically start the MCU this Zephyr image is built
for during HW boot, even if in other circumstances
this MCU would not start automatically (for ex. because
another core is meant to release its reset).

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>