Top item is no need to set a default. Also, this is no longer
possible, as some items are not available for all grub variants.

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.

This prepares the ability to write and read
the default menu item from the grubenv file.
If grub.cfg contains a line like '^default=id',
then the read value is not used.
Setting the default menu item from the grubenv file
will be indented inside the 'if' optator and therefore
does not match the '^default' pattern.

At the moment, the ability to select the kernel with which to boot is
implemented only for grub (grub-pc, grub-efi, ieee1275boot).
note that renamed STAGE1_KFLAVOUR to STAGE1_KFLAVOURS, as multiple
kernels can now be added.

Thanks underwit@
See-also: ALT bug 39632

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

It is required to set the BOOTLOADER and BOOT_TYPE variables in
grubpcboot.

Pay attention to what you are adding to the variable. No need to
add language and live_rw there! Они нужны только для refind.

Example:
@$(call set,GRUB_DEFAULT,uefi-firmware)
@$(call xport,GRUB_DEFAULT)

See features.in/grub/cfg.in/*.cfg

These changes allow you to create two independent config at the same
time or separately. It will be possible to add support for grub-pc in
the future.

The definition of the default menu item has been fixed.
Grub.cfg does not use label, uses --id.
It is assumed that --id is given last.