Add central example sketch

Note: need change the compiler parameter in platform.txt.
"compiler.c.flags=-c -std=gnu11 -mcpu=quarkse_em -mlittle-endian -g -Os ..." to
"compiler.c.flags=-c -std=gnu11 -mcpu=quarkse_em -mlittle-endian -g -O0"
"compiler.cpp.flags=-c -mcpu=quarkse_em -mlittle-endian -g -Os ..." to
"compiler.cpp.flags=-c -mcpu=quarkse_em -mlittle-endian -g -O0 ..."

Author: sgbihu

libraries/BLE/examples/central/central.ino

@@ -0,0 +1,94 @@
+
+#include "ArduinoBLE.h"
+#include "BLEAttribute.h"
+#include "BLECharacteristicImp.h"
+#include "BLEProfileManager.h"
+
+// LED pin
+#define LED_PIN   13
+
+void setup() {
+    Serial1.begin(115200);
+    Serial1.println("test---");
+    
+    // set LED pin to output mode
+    pinMode(LED_PIN, OUTPUT);
+
+    // begin initialization
+    BLE.begin();
+    Serial1.println(BLE.address());
+    
+    BLE.startScanning("LED");
+}
+
+void controlLed(BLEDevice &peripheral)
+{
+    static bool discovered = false;
+    // connect to the peripheral
+    Serial1.print("Connecting ... ");
+    Serial1.println(peripheral.address());
+
+    if (peripheral.connect())
+    {
+        Serial1.print("Connected: ");
+        Serial1.println(peripheral.address());
+    }
+    else
+    {
+        Serial1.println("Failed to connect!");
+        return;
+    }
+    
+    peripheral.discoverAttributes();
+    
+    BLECharacteristic ledCharacteristic = peripheral.characteristic("19b10101-e8f2-537e-4f6c-d104768a1214");
+    
+    if (!ledCharacteristic)
+    {
+        //peripheral.disconnect();
+        while(1)
+        {
+        Serial1.println("Peripheral does not have LED characteristic!");
+        delay(5000);
+        }
+        return;
+    } 
+      
+      
+      unsigned char ledstate = 0;
+
+    discovered = false;
+    while (peripheral.connected())
+    {
+        if (ledstate == 1)
+        {
+            ledstate = 0;
+        }
+        else
+        {
+            ledstate = 1;
+        }
+        ledCharacteristic.write(&ledstate, sizeof(ledstate));
+        delay(5000);
+    }
+    Serial1.print("Disconnected");
+    Serial1.println(peripheral.address());
+}
+
+void loop() {
+    //pr_debug(LOG_MODULE_BLE, "%s-%d",__FUNCTION__, __LINE__);
+    BLEDevice peripheral = BLE.available();
+    //pr_debug(LOG_MODULE_BLE, "%s-%d",__FUNCTION__, __LINE__);
+    if (peripheral) 
+    {
+    Serial1.println(peripheral.address());
+        BLE.stopScanning();
+        delay (1000);
+        // central connected to peripheral
+        controlLed(peripheral);
+        delay (4000);
+        BLE.startScanning("LED");
+    }
+}
+
+