Code sync and fixed compile error for Mbed OS5

Dependencies:   DHT

Revision:
2:81340dde3444
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed-os.lib	Fri Nov 08 04:22:31 2019 +0000
@@ -0,0 +1,1 @@
+https://github.com/ARMmbed/mbed-os/#3a57ec7401a77b8b98f6356a1498cb154229483f