Finish Si7021

Based on datasheet:
  https://cdn.instructables.com/ORIG/FOR/G7DY/IHXRC1XG/FORG7DYIHXRC1XG.pdf

- Add CRC8 (poly 0x31, init 0x00)
- Add Temperature and Humidity data polling.

src/mgos_si7021.c

@@ -19,6 +19,19 @@
 #include "mgos_i2c.h"
 
 // Private functions follow
+static uint8_t crc8(const uint8_t *data, int len)
+{
+  const uint8_t POLYNOMIAL=0x31;
+  uint8_t crc=0x00;
+
+  for (int j=len; j; --j ) {
+    crc ^= *data++;
+    for ( int i = 8; i; --i )
+      crc = ( crc & 0x80 ) ? (crc << 1) ^ POLYNOMIAL : (crc << 1);
+  }
+  return crc;
+}
+
 // Private functions end
 
 // Public functions follow
@@ -63,6 +76,49 @@ bool mgos_si7021_read(struct mgos_si7021 *sensor) {
   }
   // Read out sensor data here 
   //
+  uint8_t cmd = MGOS_SI7021_MEASRH_NOHOLD_CMD;
+  uint8_t data[3];
+
+  if (!mgos_i2c_write(sensor->i2c, sensor->i2caddr, &cmd, 1, false)) {
+    LOG(LL_ERROR, ("Could not write command"));
+    return false;
+  }
+  if (!mgos_i2c_read(sensor->i2c, sensor->i2caddr, data, 3, false)) {
+    LOG(LL_ERROR, ("Could not read command"));
+    return false;
+  }
+  if (data[2] != crc8(&data[0], 2)) {
+    LOG(LL_ERROR, ("CRC error on humidity data"));
+    return false;
+  }
+
+  uint16_t hum = (data[0]<<8)+data[1];
+  float humidity = hum;
+  humidity *= 125;
+  humidity /= 65536;
+  humidity -= 6;
+  sensor->humidity = humidity;
+
+  cmd=MGOS_SI7021_MEASTEMP_NOHOLD_CMD;
+  if (!mgos_i2c_write(sensor->i2c, sensor->i2caddr, &cmd, 1, false)) {
+    LOG(LL_ERROR, ("Could not write command"));
+    return false;
+  }
+  if (!mgos_i2c_read(sensor->i2c, sensor->i2caddr, data, 3, false)) {
+    LOG(LL_ERROR, ("Could not read command"));
+    return false;
+  }
+  if (data[2] != crc8(&data[0], 2)) {
+    LOG(LL_ERROR, ("CRC error on temperature data"));
+    return false;
+  }
+
+  uint16_t temp = (data[0]<<8)+data[1];
+  float temperature = temp;
+  temperature *= 175.72;
+  temperature /= 65536;
+  temperature -= 46.85;
+  sensor->temperature = temperature;
 
   LOG(LL_DEBUG, ("temperature=%.2fC humidity=%.1f%%", sensor->temperature, sensor->humidity));
   sensor->last_read_time=now;