This is an alternative for 80rescue.cfg

This variable was need for rEFInd (it no longer support). The
variable was added to grub menu, but was not in needed.
In server V, this has remained since rEFInd.

The new target adds rescue mode to the regular livecd.
live should do not be installable, since in it serious changes
that are unacceptable for a normal system.

This feature complements the live image of the second stage with installation
image-specific settings and scripted hooks.

It is recommended to connect using +live-installer to ensure that a typical set
of installer-related features is included.

Unlike the install2 feature, a separate image of the second stage of 'altinst'
is not built, but the live image is supplemented with installer packages in
order to reduce the total volume of the iso image.

Installation consists of two stages:
1. The live image is unpacked, as in livecd-install
2. Install additional packages BASE_PACKAGES and package groups THE_GROUPS as in install2.

Unlike install2, only those packages that are not in the live image are placed in the main repository. This is how the size of the iso image is reduced.

This is not required for grub >= 2.04.

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.

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.

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

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

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.