Add const defining failsafe timeout

[zdanek]

I was asked to make failsafe timeout in a const, in order to set it by "user". It would be nice if you accept my PR.

BTW does it make sense to "force" failsafe, that will cause to call enterFailsafe() in order to copy failsafe times, if you explicitly call enterFailsafe() right before?

	enterFailsafe() ;			
	LastSbusReceived = millis() - 600 ;	// Force failsafe at startup