This is not required for grub >= 2.04.

This will allow for ISO, writed on flash, disable gfxterm and
enable serial console, if necessary. For EFI only.

Example:
GRUB_TERMINAL='console serial'
GRUB_SERIAL_COMMAND='serial --unit=0 --speed=115200'

1. Didn't work before because the variable was not exported.
2. Fixed the logic broken by the introduction of support for saving
the selected item on the FAT partition.

If the ISO image has a FAT partition and the image is written
to a flash drive, then it is possible to write to this partition
of the flash drive.
This is necessary, for example, if the user
does not want to install the system, but wants to constantly work
in LiveCD mode with or without recording sessions.
In order for this code to work, you need to make changes
to the FAT partition of the image on the flash drive:
Add the 1st line to the grub.cfg file:
set pfxfat=${prefix}
Write a 1kb file grubenv.
If grub.cfg contains a line like '^default=id',
then the read value is not used.

Needed for use one grub.cfg config for all grub platforms.