Needed for bootloding with separate patition /boot.

ttyS0 is needed qemu. ttySIF0 needs to be set separately.

UBOOT_NOFDTDIR does not need more, так как bootload in QEMU uses
U-Boot/EFI.

QEMU for riscv virt machine passes its own DTB. Overriding DTB breaks
Linux loading.

ttySI0 was a name of HiFive Unleashed serial console. It was changed to
ttySIF0.

It is possible to set a timeout for automatic bootloading via a
variable.

Booting to Raspberry Pi 4 is not possible with these parameter.

When loading u-boot + extlinux.conf, needs specific arguments that
are not needed when using grub-efi

Temp fix

The entry in /etc/fstab about the mount parameters of the root
partition needs to be done not only for builds with u-boot.
But, if the record already exists, then it will not be added.
The file system type is set by the variable VM_FSTYPE.

Since packet sorting is enabled by default, then
the order in KFLAVORS has ceased to matter.
Kernels are always installed in alphabetical order.
Therefore, it makes no sense to sort by buildtime.
See-also: https://bugzilla.altlinux.org/show_bug.cgi?id=30806

Also fixed my typo. When creating a symlink, the variable
$kver was used, i.e. the entire list of kernels, not the
last value $KVER of the list.

See commit 207cec89c4515fefa2b711acd06c645ed3bd0ee8.