Include first driver: SHT31

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

include/mgos_i2c.h

@@ -12,6 +12,15 @@
 #pragma once
 
 #include "mgos.h"
+#include <stdio.h>
+#include <linux/i2c.h>
+#include <linux/i2c-dev.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/ioctl.h>
+#include <string.h>
+
 
 /* Each platform defines its own I2C connection parameters. */
 struct mgos_i2c;
@@ -113,5 +122,6 @@ void mgos_i2c_close(struct mgos_i2c *conn);
 struct mgos_i2c *mgos_i2c_get_global(void);
 
 // User contributed!
+int get_i2c_register(struct mgos_i2c *i2c, uint8_t addr, uint8_t reg, uint8_t *val);
 bool mgos_i2c_open(int busnr);