ble_mesh: Move the button component to common_components

examples/bluetooth/esp_ble_mesh/common_components/button/README.md

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+# Component: Button
+
+* This component defines a button as a well encapsulated object.
+* A button device is defined by:
+	* GPIO number on which the button is attached.
+	* Active level which decided by peripheral hardware.
+    * Trigger mode which decides whether to call serial trigger callback during pressing
+    * Serial threshold seconds which decides that serial trigger callback will be called after how many seconds' pressing
+* A button device can provide:
+    * One push event callback
+    * One release event callback
+    * One short-time tap event callback
+    * One serial trigger event callback
+    * Several long-time press event callback
+    We can set different jitter filters for all the events.
+    Once any of the long press callback is triggered, the short tap event will not be triggered.
+    This components are based on GPIO provided by idf and soft timer provided by FreeRTOS.
+    
+* To use the button device, you need to :
+	* create a button object returned by iot_button_create().
+	* Then hook different event callbacks to the button object.
+	* To free the object, you can call iot_button_delete to delete the button object and free the memory that used.
+	
+* Todo:
+    * Add hardware timer mode(because sometimes soft-timer callback function is limited)
+    
+### NOTE:
+> All the event callback function are realized by FreeRTOS soft timer APIs, the callback must follow the rule: 
+
+
+
+```
+  Button callback functions execute in the context of the timer service task.
+  It is therefore essential that button callback functions never attempt to block.
+  For example, a button callback function must not call vTaskDelay(), vTaskDelayUntil(), or specify a non zero block time when accessing a queue or a semaphore.
+```
+
+> In addition:
+> You can adjust the following macros within FreeRTOS to adjust the stack depth/queue length/task priority of the timer service.
+
+
+```
+#define configUSE_TIMERS //enable soft-timer
+#define configTIMER_TASK_PRIORITY // priority of the timers service task
+#define configQueue_LENGTH // length of timer command queue
+#define configTIMER_TASK_STACK_DEPTH // stack depth of the soft-timer
+```