boot/efi: fall back to legacy UART if EFI protocol is not available

Change-Id: I70ea16aa8842edcb22a3484cf908698c3c5026c8
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5509
Reviewed-by: Alex von Gluck IV <[email protected]>
Tested-by: Commit checker robot <[email protected]>

src/system/boot/platform/efi/serial.cpp

@@ -39,7 +39,9 @@ enum serial_register_offsets {
 	SERIAL_MODEM_STATUS			= 6,
 };
 
+#if defined(__i386__) || defined(__x86_64__)
 static uint16 sSerialBasePort = 0x3f8;
+#endif
 
 
 DebugUART* gUART = NULL;
@@ -125,6 +127,7 @@ serial_init(void)
 
 	if (status != EFI_SUCCESS || sSerial == NULL) {
 		sSerial = NULL;
+		sSerialUsesEFI = false;
 		return;
 	}