Updated for mbed-os 2019

Revision:
3:9a3c6c16dc25
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed_app.json	Thu Sep 12 13:56:16 2019 +0000
@@ -0,0 +1,3 @@
+{
+    "requires": ["bare-metal"]
+}
\ No newline at end of file