Add Message To Grub Telling User There Will Be No More Console Output

[WiteWulf]

Many users think the loader/DSM is not loading correctly as they see no screen output after the “Booting kernel” message.

Experienced users know that this is down to the lack of framebuffer in the Synology kernel, but adding a message similar to the one in Jun’s loader explicitly stating that all further output will be on the serial console would be helpful.