producer consumer for exercise 2

Revision:
0:c4887d97ba83
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed-os.lib	Wed Nov 28 16:46:47 2018 +0000
@@ -0,0 +1,1 @@
+https://github.com/armmbed/mbed-os/#bf6f2c3c6434a6de9eb9511feffa5948b3d1f20f