Turns on and of LED depending on received serial commnad "on" or "off"

Revision:
4:5ac470115649
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed-os.lib	Sun Apr 08 14:49:52 2018 +0000
@@ -0,0 +1,1 @@
+https://github.com/ARMmbed/mbed-os/#addec7ba10054be03849eff58a1d17f157391e7d