siduction-btrfs for Debian

Improves the grub menu after a rollback using snapper or manual change of the
Btrfs default subvolume.The default boot entry points to the new default
subvolume with the kernels present.
Adapted to the special requirements of siduction; a rolling relaese
based on Debian SID.

The functionality of grub-btrfs remains unaffected.

 -- Axel Konrad <emma-konrad@web.de>  Sun, 26 Feb 2023 15:10:32 +0100
