px4fmu-v5:Comment and define SAFETY LED and Button so FMU can use

  Safety Switch is HW version dependent on having an PX4IO
  So we init to a benign state with the _INIT definition
  and provide the the non _INIT one for the driver to make a run time
  decision to use it.

  We also define the FMU GPIO_BTN_SAFETY and GPIO_LED_SAFETY alias
  so the px4fmu will drrive will be built with the safety switch
  code at compile time and have runtime control via the manifest