boot: add an option to control action after SecureBoot enrollment (#36684)

boot: add an option to control action after SecureBoot enrollment (#36684)

This PR provides a new option for systemd-boot
`secure-boot-enroll-action` which allows to configure the behavior after
SecureBoot keys are enrolled.

Provides the option to either reboot or power off.

The current behavior is not changed, it will by default reboot as it did
before.

It also provides a small message about the action its going to take with
a small delay so the user can read it.