Serial output to EFI serial

[Konstantin Aladyshev]

Hello!

How to set GRUB serial output to EFI serial?

I want to set these terminal settings to GRUB, with the exception that
`serial` in this case should be EFI serial port:
```
GRUB_TERMINAL_INPUT="serial console"
GRUB_TERMINAL_OUTPUT="serial gfxterm"
```
This setting gives me:
```
grub> terminal_input
Active input terminals:
serial console
Available input terminals:
serial_efi0 serial_com0 at_keyboard
grub> terminal_output
Active output terminals:
gfxterm serial
Available output terminals:
console serial_efi0 serial_com0 spkmodem cbmemc audio
```

It is not possible to set `serial_efi0` in
GRUB_TERMINAL_INPUT/GRUB_TERMINAL_OUTPUT
(https://github.com/rhboot/grub2/blob/246d69b7ea619fc1e77dcc5960e37aea45a9808c/util/grub-mkconfig.in#L191)
Therefore I try to use GRUB_SERIAL_COMMAND:
```
GRUB_TERMINAL_INPUT="serial console"
GRUB_TERMINAL_OUTPUT="serial gfxterm"
GRUB_SERIAL_COMMAND="serial_efi0"
```
But when I do this, terminal_input and terminal_output for some reason
change to `console`:
```
grub> terminal_input
Active input terminals:
console
Available input terminals:
serial_efi0 serial_com0 serial at_keyboard
grub> terminal_output
Active output terminals:
console
Available output terminals:
serial_efi0 serial_com0 gfxterm spkmodem serial cbmemc audio
```

Why is this happening? And how to set up a serial for EFI use?

Best regards,
Konstantin Aladyshev