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

`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

PUBLISHED_FROM=fb548dc351b24de0f0ceb1b0f974be3b495264d3

src/mgos_core.c

@@ -37,7 +37,6 @@ static const char *dt = STRINGIZE(MGOS_ROOT_DEVTAB);
 #include "mgos_gpio_internal.h"
 #include "mgos_net_internal.h"
 #include "mgos_sys_config_internal.h"
-#include "mgos_time_internal.h"
 
 extern int mg_ssl_if_mbed_random(void *ctx, unsigned char *buf, size_t len);
 
@@ -78,7 +77,6 @@ bool mgos_core_init(void) {
   srand(seed);
 
   mgos_event_register_base(MGOS_EVENT_SYS, "mos");
-  mgos_uptime_init();
 
   r = mgos_net_init();
   if (r != MGOS_INIT_OK) {