Add BME280 Sensor

- Plumb BME280 all the way through to Prometheus.
- Add README detailing the diff between BME280 and BMP280
- Refactor source files to be *_drv.c to avoid naming clashes with
  upstream libraries.

src/dht_drv.c

@@ -0,0 +1,111 @@
+#include "mgos.h"
+
+#ifdef MGOS_HAVE_DHT
+#include "mgos_dht.h"
+#include "mgos_config.h"
+#include "mgos_prometheus_metrics.h"
+#include "mgos_prometheus_sensors.h"
+
+#define MAX_DHT 8
+
+struct dht_sensor {
+  struct mgos_dht *dht;
+  float temp;
+  float humidity;
+  uint8_t gpio;
+  uint8_t idx;
+};
+
+static struct dht_sensor *s_dht_sensor[MAX_DHT];
+static int s_num_dht = 0;
+
+static void dht_prometheus_metrics(struct mg_connection *nc, void *user_data) {
+  int i;
+
+  for (i=0; i<s_num_dht; i++) {
+    mgos_prometheus_metrics_printf(nc, GAUGE,
+      "temperature", "Temperature in Celcius",
+      "{sensor=\"%d\",type=\"DHT\"} %f", i, s_dht_sensor[i]->temp);
+    mgos_prometheus_metrics_printf(nc, GAUGE,
+      "humidity", "Relative humidity percentage",
+      "{sensor=\"%d\",type=\"DHT\"} %f", i, s_dht_sensor[i]->humidity);
+
+  }
+
+  (void) user_data;
+}
+
+static void dht_timer_cb(void *user_data) {
+  struct dht_sensor *dht_sensor = (struct dht_sensor *)user_data;
+  double start;
+  uint32_t usecs=0;
+
+  if (!dht_sensor) return;
+
+  start=mgos_uptime();
+  dht_sensor->temp = mgos_dht_get_temp(dht_sensor->dht);
+  dht_sensor->humidity = mgos_dht_get_humidity(dht_sensor->dht);
+  usecs=1000000*(mgos_uptime()-start);
+  LOG(LL_DEBUG, ("DHT sensor=%u gpio=%u temp=%.2fC humidity=%.0f%% usecs=%u", dht_sensor->idx, dht_sensor->gpio, dht_sensor->temp, dht_sensor->humidity, usecs));
+}
+
+static bool dht_sensor_create(int pin, enum dht_type type) {
+  struct dht_sensor *dht_sensor = calloc(1, sizeof(struct dht_sensor));
+  if (s_num_dht == MAX_DHT) {
+    LOG(LL_ERROR, ("No more sensor slots available (%d added)", MAX_DHT));
+    free(dht_sensor);
+    return false;
+  }
+
+  dht_sensor->dht=mgos_dht_create(pin, type);
+  if (!dht_sensor->dht) {
+    LOG(LL_ERROR, ("Could not create DHT sensor on pin %d", pin));
+    free(dht_sensor);
+    return false;
+  }
+  dht_sensor->gpio=pin;
+  dht_sensor->idx=s_num_dht;
+  s_dht_sensor[dht_sensor->idx] = dht_sensor;
+  s_num_dht++;
+  mgos_set_timer(mgos_sys_config_get_sensors_dht_period()*1000, true, dht_timer_cb, (void*)dht_sensor);
+  return true;
+}
+
+float mgos_prometheus_sensors_dht_get_temp(uint8_t idx) {
+  if (idx>=s_num_dht)
+    return NAN;
+  if (!s_dht_sensor[idx])
+    return NAN;
+  return s_dht_sensor[idx]->temp;
+}
+
+float mgos_prometheus_sensors_dht_get_humidity(uint8_t idx) {
+  if (idx>=s_num_dht)
+    return NAN;
+  if (!s_dht_sensor[idx])
+    return NAN;
+  return s_dht_sensor[idx]->humidity;
+}
+
+void dht_drv_init() {
+  char *tok;
+
+  memset(s_dht_sensor, 0, sizeof (struct mgos_dht_sensor *) * MAX_DHT);
+  tok = strtok((char *)mgos_sys_config_get_sensors_dht_gpio(), ", ");
+  while (tok) {
+    int gpio;
+    gpio = atoi(tok);
+    tok=strtok(NULL, ", ");
+    dht_sensor_create(gpio, AM2302);
+    mgos_msleep(250);
+  }
+
+  if (s_num_dht>0)
+    mgos_prometheus_metrics_add_handler(dht_prometheus_metrics, NULL);
+}
+
+#else
+void dht_drv_init() {
+  LOG(LL_ERROR, ("DHT disabled, include library in mos.yml to enable"));
+}
+#endif