`mgos_uptime_micros()` provides a reliable, microsecond-accurate monotonic counter on all our supported platforms:

 * On ESP32 we have esp_timer_get_time()
 * On ESP8266 we have system_get_time() (plus overflow handling)
 * On ARM-based platforms we use FreeRTOS xTaskGetTickCount() + SYSTICK for fractional ticks

CL: Add mgos_uptime_micros(), fix sleep issues on CC32x and STM32

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