copy of rtc example

Revision:
1:a474ab5e98f9
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed-os.lib	Fri Apr 21 19:43:43 2017 +0000
@@ -0,0 +1,1 @@
+https://github.com/ARMmbed/mbed-os/#42be5c01a7f91292d5e27124ad9584236025f6ab