updated for mbed-os 5.5

Fork of Task121 by Nicholas Outram

Revision:
12:991c54e21f5f
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/.gitignore	Thu Jul 16 10:10:13 2020 +0100
@@ -0,0 +1,3 @@
+BUILD
+.mbed
+mbed-os