MBED NRF51 Arduboy port

Revision:
0:649b2fe69f16
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed-os.lib	Thu Jan 05 18:37:50 2017 +0000
@@ -0,0 +1,1 @@
+https://github.com/ARMmbed/mbed-os/#0789928ee7f2db08a419fa4a032fffd9bd477aa7