Using string to identify region rather than number

Using EU868 as the region identifier by default and hence giving away the
use of an identification number.

Author: Hasnain Virk

mbed_app.json

@@ -32,7 +32,7 @@
             "platform.default-serial-baud-rate": 115200,
             "lora.over-the-air-activation": true,
             "lora.duty-cycle-on": true,
-            "lora.phy": 0,
+            "lora.phy": "EU868",
             "lora.device-eui": "{ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 }",
             "lora.application-eui": "{ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 }",
             "lora.application-key": "{ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 }"