Fixes #37562 - Fix local disk boot over network

[jloeser]

The assumption was that exit 1 in GRUB2 triggers a boot from the next bootdevice by the firmware and that the chainloader command is not working at all when SecureBoot is enabled (lockdown=y).

These assumptions seems to be wrong. It looks like that distribution vendors patch GRUB2 differently which results in different behavior affecting these assumptions. Some support chainloader command, some do simply end up in the BIOS menu when using exit 1.

As an alternative we can do a "chainload light" and only load the GRUB2 configuration file from local disk. This means that the PXE booted GRUB2 boots the actual kernel from local disk.

For successful SecureBoot verification, the following changes are required:

https://github.com/theforeman/foreman/pull/9864

The proposed solution would also work when SecureBoot is disabled, however to avoid side effects I propose to only boot next device if SecureBoot is enabled (GRUB2 variable lockdown=y [2]).