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/veml6075_drv.c

@@ -0,0 +1,52 @@
+#include "mgos.h"
+
+#ifdef MGOS_HAVE_VEML6075_I2C
+#include "mgos_veml6075.h"
+#include "mgos_config.h"
+#include "mgos_prometheus_metrics.h"
+#include "mgos_prometheus_sensors.h"
+
+static struct mgos_veml6075 *s_veml6075;
+
+static void veml6075_prometheus_metrics(struct mg_connection *nc, void *user_data) {
+  mgos_prometheus_metrics_printf(nc, GAUGE,
+    "UV", "Ultra Violet light intensity, in sensor counts",
+    "{band=\"UVA\",type=\"VEML6075\", sensor=\"0\"} %f", mgos_veml6075_getUVA(s_veml6075));
+  mgos_prometheus_metrics_printf(nc, GAUGE,
+    "UV", "Ultra Violet light intensity, in sensor counts",
+    "{band=\"UVB\",type=\"VEML6075\", sensor=\"0\"} %f", mgos_veml6075_getUVB(s_veml6075));
+  mgos_prometheus_metrics_printf(nc, GAUGE,
+    "UVIndex", "2: Low, 5.5 Moderate, 7.5 High, 10.5 Very High, else Extreme",
+    "{sensor=\"0\",type=\"VEML6075\"} %f", mgos_veml6075_getUVIndex(s_veml6075));
+
+  (void) user_data;
+}
+
+static void veml6075_timer_cb(void *user_data) {
+  double start;
+  uint32_t usecs=0;
+  float uva, uvb, uvindex;
+
+  start=mgos_uptime();
+  uva=mgos_veml6075_getUVA(s_veml6075);
+  uvb=mgos_veml6075_getUVB(s_veml6075);
+  uvindex=mgos_veml6075_getUVIndex(s_veml6075);
+  usecs=1000000*(mgos_uptime()-start);
+  LOG(LL_INFO, ("VEML6075 sensor=0 uva=%.1f uvb=%.1f uvindex=%.2f usecs=%u", uva, uvb, uvindex, usecs));
+
+  (void) user_data;
+}
+
+void veml6075_drv_init() {
+  s_veml6075 = mgos_veml6075_create(mgos_sys_config_get_sensors_veml6075_i2caddr());
+  if (s_veml6075) {
+    mgos_set_timer(mgos_sys_config_get_sensors_veml6075_period()*1000, true, veml6075_timer_cb, NULL);
+    mgos_prometheus_metrics_add_handler(veml6075_prometheus_metrics, NULL);
+  }
+}
+
+#else
+void veml6075_drv_init() {
+  LOG(LL_ERROR, ("VEML6075 disabled, include library in mos.yml to enable"));
+}
+#endif