NuMaker bootloader with SD mode

Revision:
3:3f3c7e764947
Child:
5:795f3afc4a43
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed-os.lib	Tue Dec 26 13:26:53 2017 +0800
@@ -0,0 +1,1 @@
+https://github.com/ARMmbed/mbed-os/#8b54959f5bca08a8ad44a701bafd634afbb38b03