Check RTC LSE(External Xtal, 32.768kHz) mode

Dependencies:   mbed CheckRTC

Please refer follow links.
My notebook
http://developer.mbed.org/users/kenjiArai/notebook/nucleo-series-clock-structure-and-xtal-oscillation/
Q&A for RTC Xtal
http://developer.mbed.org/questions/4531/RTC-External-Crystal-Nucleo/

Revision:
2:8b6a8cfa173a
Parent:
1:eaa3c3f36c03
Child:
4:56a0cbad89b0
--- a/mbed.bld	Sat Oct 25 05:42:45 2014 +0000
+++ b/mbed.bld	Sat Nov 01 00:35:50 2014 +0000
@@ -1,1 +1,1 @@
-http://mbed.org/users/mbed_official/code/mbed/builds/552587b429a1
\ No newline at end of file
+http://mbed.org/users/mbed_official/code/mbed/builds/031413cf7a89
\ No newline at end of file