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>

boot/boot_serial/src/boot_serial.c

@@ -613,6 +613,7 @@ boot_serial_start(const struct boot_uart_funcs *f)
 
     off = 0;
     while (1) {
+        MCUBOOT_CPU_IDLE();
         rc = f->read(in_buf + off, sizeof(in_buf) - off, &full_line);
         if (rc <= 0 && !full_line) {
             continue;