We propose to switch to loading the local disk's GRUB2 configuration in case SecureBoot is disabled.

To support SecureBoot for arbitrary operating systems, support for distribution vendor specific boot files will be added by https://github.com/theforeman/foreman/pull/9864 and the related PRs.
Distribution vendors patch GRUB2 differently which results in

• different behavior when using `exit 1` (see: https://github.com/theforeman/foreman/pull/10207) and
• differences in the support of the `connectefi` command (available on EL only), which affects chainloading from local disks using the `chainloader` command (see: https://github.com/theforeman/foreman/pull/10126).

Therefore, in case distribution vendor specific boot files are set up for the operating system of a host, using the `chainloader` command may lead to problems during boot in case the distribution vendor specific GRUB2 doesn't support the `connectefi` command - which is at least true for non EL systems.

For SecureBoot enabled hosts this has already been fixed by switching to loading the local disk's GRUB2 configuration (https://github.com/theforeman/foreman/pull/10207). Now we want to introduce this also for the case that SecureBoot is disabled.
This standardizes the procedure under UEFI for both cases (SecureBoot enabled and disabled) and also obsoletes using the `connectefi` command.