Include first driver: SHT31

Make I2C reads blocking w/ timeout
Add SHT31 driver, roughly :)
Fix mg_time() to return a double.

src/mgos_mock.c

@@ -2,6 +2,7 @@
  */
 
 #include "mgos_mock.h"
+#include <unistd.h>
 
 int _mgos_timers = 0;
 
@@ -31,14 +32,18 @@ int log_print_prefix(enum cs_log_level l, const char *func, const char *file) {
   return 1;
 }
 
-float mg_time() {
-  struct timeval tod;
-  float time;
+double mg_time() {
+  struct timespec ts;
+  double ret;
 
-  gettimeofday(&tod, NULL);
+  clock_gettime(CLOCK_REALTIME_COARSE, &ts);
 
-  time = tod.tv_sec;
-  time += tod.tv_usec/1000000;
+  ret = (double) ts.tv_sec;
+  ret += ((double)ts.tv_nsec)/1000000000;
 
-  return time;
+  return ret;
+}
+
+void mgos_usleep(uint32_t usecs) {
+  usleep(usecs);
 }