Added BMP180 and I2C driver and example

Fixed reference to unknown environment var
Moved drivers into extras folder and added additional documentations.
Changed LOCAL to static and renamed DEBUG to BMP180_DEBUG

examples/bmp180_i2c/Makefile

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+PROGRAM=BMP180_Reader
+EXTRA_COMPONENTS = extras/i2c extras/bmp180
+include ../../common.mk

examples/bmp180_i2c/README.md

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+# I2C / BMP180 Example
+
+This example references two addtional drivers [i2c](https://github.com/kanflo/esp-open-rtos-driver-i2c) and [bmp180](https://github.com/Angus71/esp-open-rtos-driver-bmp180), which are provided in the `../../extras` folder.
+
+If you plan to use one or both of this drivers in your own projects, please check the main development pages for updated versions or reported issues.
+
+To run this example connect the BMP085/BMP180 SCL to GPIO0 and SDA to GPIO2.

examples/bmp180_i2c/bmp180_i2c.c

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+/* Simple example for I2C / BMP180 / Timer & Event Handling
+ *
+ * This sample code is in the public domain.
+ */
+#include "espressif/esp_common.h"
+#include "espressif/sdk_private.h"
+
+#include "FreeRTOS.h"
+#include "task.h"
+#include "timers.h"
+#include "queue.h"
+
+// BMP180 driver
+#include "bmp180/bmp180.h"
+
+#define MY_EVT_TIMER  0x01
+#define MY_EVT_BMP180 0x02
+
+#define SCL_PIN GPIO_ID_PIN((0))
+#define SDA_PIN GPIO_ID_PIN((2))
+
+typedef struct
+{
+    uint8_t event_type;
+    bmp180_result_t bmp180_data;
+} my_event_t;
+
+// Communication Queue
+static xQueueHandle mainqueue;
+static xTimerHandle timerHandle;
+
+// Own BMP180 User Inform Implementation
+bool bmp180_i2c_informUser(const xQueueHandle* resultQueue, uint8_t cmd, bmp180_temp_t temperatue, bmp180_press_t pressure)
+{
+    my_event_t ev;
+
+    ev.event_type = MY_EVT_BMP180;
+    ev.bmp180_data.cmd = cmd;
+    ev.bmp180_data.temperatue = temperatue;
+    ev.bmp180_data.pressure = pressure;
+
+    return (xQueueSend(*resultQueue, &ev, 0) == pdTRUE);
+}
+
+// Timer call back
+static void bmp180_i2c_timer_cb(xTimerHandle xTimer)
+{
+    my_event_t ev;
+    ev.event_type = MY_EVT_TIMER;
+
+    xQueueSend(mainqueue, &ev, 0);
+}
+
+// Check for communiction events
+void bmp180_task(void *pvParameters)
+{
+    // Received pvParameters is communication queue
+    xQueueHandle *com_queue = (xQueueHandle *)pvParameters;
+
+    printf("%s: Started user interface task\n", __FUNCTION__);
+
+    while(1)
+    {
+        my_event_t ev;
+
+        xQueueReceive(*com_queue, &ev, portMAX_DELAY);
+
+        switch(ev.event_type)
+        {
+        case MY_EVT_TIMER:
+            printf("%s: Received Timer Event\n", __FUNCTION__);
+
+            bmp180_trigger_measurement(com_queue);
+            break;
+        case MY_EVT_BMP180:
+            printf("%s: Received BMP180 Event temp:=%ld.%d°C press=%ld.%02ldhPa\n", __FUNCTION__, \
+                       (int32_t)ev.bmp180_data.temperatue, abs((int32_t)(ev.bmp180_data.temperatue*10)%10), \
+                       ev.bmp180_data.pressure/100, ev.bmp180_data.pressure%100 );
+            break;
+        default:
+            break;
+        }
+    }
+}
+
+// Setup HW
+void user_setup(void)
+{
+    // Set UART Parameter
+    sdk_uart_div_modify(0, UART_CLK_FREQ / 115200);
+
+    // Give the UART some time to settle
+    sdk_os_delay_us(500);
+}
+
+void user_init(void)
+{
+    // Setup HW
+    user_setup();
+
+    // Just some infomations
+    printf("\n");
+    printf("SDK version : %s\n", sdk_system_get_sdk_version());
+    printf("GIT version : %s\n", GITSHORTREV);
+
+    // Use our user inform implementation
+    bmp180_informUser = bmp180_i2c_informUser;
+
+    // Init BMP180 Interface
+    bmp180_init(SCL_PIN, SDA_PIN);
+
+    // Create Main Communication Queue
+    mainqueue = xQueueCreate(10, sizeof(my_event_t));
+
+    // Create user interface task
+    xTaskCreate(bmp180_task, (signed char *)"bmp180_task", 256, &mainqueue, 2, NULL);
+
+    // Create Timer (Trigger a measurement every second)
+    timerHandle = xTimerCreate((signed char *)"BMP180 Trigger", 1000/portTICK_RATE_MS, pdTRUE, NULL, bmp180_i2c_timer_cb);
+
+    if (timerHandle != NULL)
+    {
+        if (xTimerStart(timerHandle, 0) != pdPASS)
+        {
+            printf("%s: Unable to start Timer ...\n", __FUNCTION__);
+        }
+    }
+    else
+    {
+        printf("%s: Unable to create Timer ...\n", __FUNCTION__);
+    }
+}