building external library fails with yield not implemented errors

[DhruvaG2000]

Fix is to just:

diff --git a/cores/arduino/zephyrCommon.cpp b/cores/arduino/zephyrCommon.cpp
index a9c053b..7e47fcc 100644
--- a/cores/arduino/zephyrCommon.cpp
+++ b/cores/arduino/zephyrCommon.cpp
@@ -6,6 +6,9 @@

 #include "Arduino.h"

+void yield(void) {}
+//empty
+
 /*
  *  The ACTIVE_HIGH flag is set so that A low physical
  *  level on the pin will be interpreted as value 0.