Add function to test if serial configuration is in progress

This was formely tested all over using testing for the state of
gSerialConfigCountDown_500ms in a couple of ways. This logic have been
extracted into a function to make the code more readable.
Also testing it with > 0 was a bit misleading as the variable is
unsigned so tesing with == and != is enough.

misc.h

@@ -333,4 +333,6 @@ unsigned long StrToUL(const char * str);
 
 void FUNCTION_NOP();
 
+inline bool SerialConfigInProgress() { return gSerialConfigCountDown_500ms != 0; }
+
 #endif