check program for USB Serial

Dependencies:   USBDevice

Revision:
1:637d33b55dfe
diff -r 941291e23892 -r 637d33b55dfe mbed-os.lib
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed-os.lib	Mon Apr 16 10:48:20 2018 +0000
@@ -0,0 +1,1 @@
+https://github.com/ARMmbed/mbed-os/#f9ee4e849f8cbd64f1ec5fdd4ad256585a208360