Doc update and using PlatformMutex for tracing

We have updated the doc to reflect that 6K size can be achieved if you
drop RTOS and use new-lib-nano.

Instead of using mutex directly, we should use PlatformMutex so that if
RTOS is not being used, adding trace may not end up in error.

Author: Hasnain Virk

README.md

@@ -141,6 +141,19 @@ Mbed LoRaWANStack initialized
 
 ```
 
+## [Optional] Adding trace library
+To enable Mbed trace, add to your `mbed_app.json` the following fields:
+
+```json
+    "target_overrides": {
+        "*": {
+            "target.features_add": ["COMMON_PAL"],
+            "mbed-trace.enable": true
+            }
+     }
+```
+The trace is disabled by default to save RAM.
+
 ## [Optional] Memory optimization 
 
 Using `Arm CC compiler` instead of `GCC` reduces `3K` of RAM. Currently the application takes about `15K` of static RAM with Arm CC, which spills over for the platforms with `20K` of RAM because you need to leave space, about `5K`, for dynamic allocation. So if you reduce the application stack size, you can barely fit into the 20K platforms.
@@ -153,6 +166,7 @@ For example, add the following into `config` section in your `mbed_app.json`:
 }
 ```
 
-Essentially you can make the whole application with Mbed LoRaWAN stack in 6K if you drop the RTOS from Mbed OS. 
+Essentially you can make the whole application with Mbed LoRaWAN stack in 6K if you drop the RTOS from Mbed OS and use a smaller standard C/C++ library like new-lib-nano. Please find instructions [here](https://os.mbed.com/blog/entry/Reducing-memory-usage-with-a-custom-prin/).
+ 
 
 For more information, please follow this [blog post](https://os.mbed.com/blog/entry/Reducing-memory-usage-by-tuning-RTOS-con/).

trace_helper.cpp

@@ -29,13 +29,13 @@ mbed::Serial pc(USBTX, USBRX, MBED_CONF_PLATFORM_DEFAULT_SERIAL_BAUD_RATE);
  */
 #if defined(FEATURE_COMMON_PAL)
 
-    #include "rtos/Mutex.h"
+    #include "platform/PlatformMutex.h"
     #include "mbed_trace.h"
 
     /**
      * Local mutex object for synchronization
      */
-    static rtos::Mutex mutex;
+    static PlatformMutex mutex;
 
     static void serial_lock();
     static void serial_unlock();