px4io: set safety on before going into bootloader (#4860)

Sometimes when flashing new firmware, the IO update fails because safety
is off. In this case, we should set safety on first before putting the
IO board into bootloader mode.

src/drivers/px4io/px4io.cpp

@@ -699,6 +699,12 @@ PX4IO::init()
 		// be due to mismatched firmware versions and we want
 		// the startup script to be able to load a new IO
 		// firmware
+
+		// If IO has already safety off it won't accept going into bootloader mode,
+		// therefore we need to set safety on first.
+		io_reg_set(PX4IO_PAGE_SETUP, PX4IO_P_SETUP_FORCE_SAFETY_ON, PX4IO_FORCE_SAFETY_MAGIC);
+
+		// Now the reboot into bootloader mode should succeed.
 		io_reg_set(PX4IO_PAGE_SETUP, PX4IO_P_SETUP_REBOOT_BL, PX4IO_REBOOT_BL_MAGIC);
 		return -1;
 	}