esp32: Use system_get_rtc_time for time_ticks_us().
esp32: Use system_get_rtc_time for time_ticks_us().

This method is also used internnaly for the LoRa timestamps,
if we use a different one here, the LoRaWAN nano-gateway fails to
send the downlinks.

husigeza/pycom-micropython-sigfox,husigeza/pycom-micropython-sigfox,husigeza/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox,husigeza/pycom-micropython-sigfox,husigeza/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox