Making items configurable through json

Following the fashion of other mbed-os examples, we are
making cellular items like pin code, APN etc configurable through json file.

Author: Hasnain Virk

main.cpp

@@ -7,22 +7,30 @@
 #define TCP 1
 
 // SIM pin code goes here
-#define PIN_CODE    "1234"
+#ifndef MBED_CONF_APP_SIM_PIN_CODE
+# define MBED_CONF_APP_SIM_PIN_CODE    "1234"
+#endif
 
-// Network credentials like APN go here, e.g.,
-// "apn, username, password"
-#define CREDENTIALS "internet"
+#ifndef MBED_CONF_APP_APN
+# define MBED_CONF_APP_APN         "internet"
+#endif
+#ifndef MBED_CONF_APP_USERNAME
+# define MBED_CONF_APP_USERNAME    NULL
+#endif
+#ifndef MBED_CONF_APP_PASSWORD
+# define MBED_CONF_APP_PASSWORD    NULL
+#endif
 
 // Number of retries /
 #define RETRY_COUNT 3
 
 // CellularInterface object
 OnboardCellularInterface iface;
 
-// NIST ntp hostname
+// Echo server hostname
 const char *host_name = "echo.u-blox.com";
 
-// NIST ntp port
+// Echo server port (same for TCP and UDP)
 const int port = 7;
 
 /**
@@ -135,10 +143,10 @@ int main()
 {
     iface.modem_debug_on(MBED_CONF_APP_MODEM_TRACE);
     /* Set Pin code for SIM card */
-    iface.set_sim_pin(PIN_CODE);
+    iface.set_sim_pin(MBED_CONF_APP_SIM_PIN_CODE);
 
     /* Set network credentials here, e.g., APN*/
-    iface.set_credentials(CREDENTIALS);
+    iface.set_credentials(MBED_CONF_APP_APN, MBED_CONF_APP_USERNAME, MBED_CONF_APP_PASSWORD);
 
     printf("\n\nmbed-os-example-cellular, Connecting...\n");
 

mbed_app.json

@@ -1,7 +1,26 @@
 {
-	"config": {
-		"sock-type": "UDP",
-		"modem_trace": false
+    "config": {
+        "sock-type": "UDP",
+        "modem_trace": {
+            "help": "Turns AT command trace on/off from the cellular modem, defaults to off",
+            "value": false
+        },
+        "sim-pin-code": {
+            "help": "SIM PIN code",
+            "value": "\"1234\""
+        },
+        "apn": {
+            "help": "The APN string to use for this SIM/network, set to 0 if none",
+            "value": "\"internet\""
+        },
+        "username": {
+            "help": "The user name string to use for this APN, set to zero if none",
+            "value": 0
+        },
+        "password": {
+            "help": "The password string to use for this APN, set to 0 if none",
+            "value": 0
+        }
 	},
     "target_overrides": {
         "*": {