sim: Fix sys_kernel re-importing host time module

By importing `_time` in `freertos_sleep()`, the host time module was
imported as `time` again as well, breaking all later code using e.g.
`time.ticks_ms()`.

Fix this by using `time` in `sys_kernel.freertos_sleep()` as well.

sim/fakes/sys_kernel.py

@@ -24,9 +24,9 @@ def usb_console_active():
 
 
 def freertos_sleep(ms):
-    import _time
+    import time
 
-    _time.sleep(ms / 1000.0)
+    time.sleep(ms / 1000.0)
 
 
 def i2c_scan():