updated for mbed-os 5.5

Fork of Task121 by Nicholas Outram

Revision:
11:d7337ffe7f85
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/.hgignore	Thu Jul 16 09:47:03 2020 +0100
@@ -0,0 +1,3 @@
+^BUILD$
+^.mbed$
+^mbed-os$