boot/boot_serial: call idle for reducing power in single thread

Sine zephyr https://github.com/zephyrproject-rtos/zephyr/pull/34279


was merged there is no silent idle thread created automatically while
CONFIG_MULTITHREADING=n. Since that any single thread application
needs to call k_cpu_idle() (wrapped by MCUBOOT_CPU_IDLE)
by itself for entering idle mode, which allows for reduction
power consumption.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>