mbed_app.json configured to operate on senet

To enable cayenne, edit mbed_app.json, assign pins for desired function

Example:

        "cayenne": {
            "help": "compile main_cayenne.cpp instead of main.cpp",
            "value": 1
        },
        "analog_in_pin": {
            "help": "PA_0, PA_4, PA_5",
            "value": "PA_0"
        },
        "analog_out_pin": {
            "help": "PA_0, PB_5, PB_10, PB_11",
            "value": "PB_11"
        },
        "_digital_out_pin": {
            "help": "",
            "value": ""
        }
Revision:
12:5015dfead3f2
Parent:
2:dc95ac6d6d4e
Child:
26:f07f5febf97f
--- a/README.md	Wed Apr 11 06:30:22 2018 +0100
+++ b/README.md	Wed Apr 18 06:30:24 2018 +0100
@@ -152,7 +152,9 @@
             }
      }
 ```
-The trace is disabled by default to save RAM.
+The trace is disabled by default to save RAM and reduce main stack usage (see chapter Memory optimization).
+
+**Please note that some targets with small RAM size (e.g. DISCO_L072CZ_LRWAN1 and MTB_MURATA_ABZ) mbed traces cannot be enabled without increasing the default** `"main_stack_size": 1024`**.**
 
 ## [Optional] Memory optimization