Silence GCC warning by initializing the local variable.

GCC 6.3 is unable to trace all code paths that lead to
this variable being left uninitialized and correlate that
to function return values.

sys/boot/efi/boot1/boot1.c

@@ -529,6 +529,7 @@ probe_handle_status(EFI_HANDLE h, EFI_DEVICE_PATH *imgpath)
 	EFI_STATUS status;
 	BOOLEAN preferred;
 
+	preferred = FALSE;
 	status = probe_handle(h, imgpath, &preferred);
 	
 	DPRINTF("probe: ");