Add app.dht_gpio flag to find DHT sensors

Parse this comma separated string to find GPIO pins on which we must
enable DHT sensors.  Seperators are space and comma.
Can be empty, single GPIO pins, or a list:
app.dht_gpio=""
app.dht_gpio="5"
app.dht_gpio="5,4,14"
app.dht_gpio="5 4 14"
app.dht_gpio="5, 4 14"

src/dht.c

@@ -3,6 +3,7 @@
 #ifdef MGOS_HAVE_DHT
 #include "main.h"
 #include "mgos_dht.h"
+#include "mgos_config.h"
 #include "mqtt.h"
 
 #define MAX_DHT 8
@@ -44,12 +45,17 @@ static bool dht_create(int pin, enum dht_type type) {
 }
 
 void dht_init() {
+  char *tok;
+
   memset(s_dht, 0, sizeof (struct mgos_dht *) * MAX_DHT);
-  dht_create(5, AM2302);
-  mgos_msleep(250);
-  dht_create(4, AM2302);
-  mgos_msleep(250);
-  dht_create(14, AM2302);
+  tok = strtok((char *)mgos_sys_config_get_app_dht_gpio(), ", ");
+  while (tok) {
+    int gpio;
+    gpio = atoi(tok);
+    tok=strtok(NULL, ", ");
+    dht_create(gpio, AM2302);
+    mgos_msleep(250);
+  }
 }
 
 #else