The new boot/grub package implements boot.Protocol for GRUB, based on
the three environment variables documented below.

This implementation is compatible with both the legacy and the EFI
version of GRUB. It does not rely on any EFI-specific features. The
internal protocol between the userspace and the bootloader is organized
with three GRUB environment variables.

There is one variable which needs to be permanently present and two
variables which participate with the try-boot mechanism. The
SYSOTA_BOOT_ACTIVE variable must be either set to "A" or "B" and define
the slot which is booted into outside of the update process. When an
update is attempted SysOTA sets SYSOTA_BOOT_NEXT to the name of the
slot, again either "A" or "B" that the bootloader should boot next. The
bootloader script guarantees that this slot is only booted into once by
deleting the variable before transferring control.  Before erasing the
variable, the value is copied to SYSOTA_BOOT_TESTED. Back in userspace,
SysOTA can commit or rollback the update transaction by looking at this
variable and either erasing it (rollback) or copying it to
SYSOTA_BOOT_ACTIVE (commit).

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