boot_serial: allow to build when CONFIG_MULTITHREADING=n

For zephyr-rtos:
K_sleep() is not available when multithreading is disabled.
Let's use k_busy_wait() in that case.

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

Author: nvlsianpu

boot/boot_serial/src/boot_serial.c

@@ -436,7 +436,11 @@ bs_reset(char *buf, int len)
     bs_empty_rsp(buf, len);
 
 #ifdef __ZEPHYR__
+#ifdef CONFIG_MULTITHREADING
     k_sleep(K_MSEC(250));
+#else
+    k_busy_wait(250000);
+#endif
     sys_reboot(SYS_REBOOT_COLD);
 #else
     os_cputime_delay_usecs(250000);