grub: replace stub boot script with sysota-compatible logic

This script implements proper A/B update with automatic rollback.
Refer to sysota.grub(7) manual page for details.

The script should become built-into grub EFI executable but that will be
pursued separately.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>