Adding description how to modify mbed-os-to-arduino in order to build mbed libraries with debug flags enabled

Author: aentinger

README.md

@@ -27,6 +27,20 @@ Open Arduino IDE; you should now see three new targets under `MBED boards` label
 Adding a target is a mostly automatic procedure that involves running https://github.com/bcmi-labs/ArduinoCore-mbed/blob/master/mbed-os-to-arduino after setting `BOARDNAME` and `ARDUINOCORE` env variables.
 Actions marked as TODO must be executed manually.
 
+### How to build a debug version of the Arduino mbed libraries
+* Modify `mbed-os-to-arduino `
+```diff
+mbed_compile () {
+-       PROFILE_FLAG=""
+        if [ x"$PROFILE" != x ]; then
+                PROFILE_FLAG=--profile="$ARDUINOVARIANT"/conf/profile/$PROFILE.json
+                export PROFILE=-${PROFILE^^}
++       else
++               export PROFILE="-DEBUG"
++               PROFILE_FLAG="--profile=debug"
+        fi
+```
+
 ## Using this core as an mbed library
 
 You can use this core as a standard mbed library; all APIs are under `arduino` namespace (so they must be called like `arduino::digitalWrite()` )