Add multiple kernels support and initrd.img support instead full.cz.

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.