Add DHT support.

Very rough stab just to test the waters -- this will need some
more work.

For now, hard code 4,5,15 pins (as per Sonoff SV), but before
rolling this out, add a config flag for this behavior.

Neat trick -- the presence of dht library in mos.yml triggers
inclusion of the code via define -DMGOS_HAVE_DHT=1

src/dht.c

@@ -0,0 +1,43 @@
+#include "mgos.h"
+
+#ifdef MGOS_HAVE_DHT
+#include "main.h"
+#include "mgos_dht.h"
+#include "mqtt.h"
+static void dht_cb(void *ud) {
+  struct mgos_dht *dht = (struct mgos_dht *)ud;
+  float t, h;
+
+  if (!dht) {
+    LOG(LL_ERROR, ("No DHT handle to work with!"));
+    return;
+  }
+  t = mgos_dht_get_temp(dht);
+  h = mgos_dht_get_humidity(dht);
+
+  mqtt_publish_stat("dht", "{temp=%.2f, humidity=%.1f}", t, h);
+}
+
+static void dht_create(int pin, enum dht_type type) {
+  struct mgos_dht *dht;
+  dht=mgos_dht_create(pin, type);
+  if (!dht) {
+    LOG(LL_ERROR, ("Could not create DHT sensor on pin %d", pin));
+    return;
+  }
+  mgos_set_timer(5000, true, dht_cb, (void*)dht);
+}
+
+void dht_init() {
+  dht_create(5, AM2302);
+  mgos_msleep(250);
+  dht_create(4, AM2302);
+  mgos_msleep(250);
+  dht_create(14, AM2302);
+}
+
+#else
+void dht_init() {
+  LOG(LL_INFO, ("DHT disabled, include library in mos.yml to enable"));
+}
+#endif

src/main.c

@@ -7,5 +7,6 @@ enum mgos_app_init_result mgos_app_init(void) {
   channel_init(mgos_sys_config_get_app_config());
   mqtt_init();
   rpc_init();
+  dht_init();
   return MGOS_APP_INIT_SUCCESS;
 }