release: ensure 'prepare-boot' runs after 'mark-successful-boot'

[etungsten]

Issue number:

Closes #

Description of changes:

    release: ensure 'prepare-boot' runs after 'mark-successful-boot'
    
    To prevent prairiedog from reading the GPT table while signpost is still
    writing to it during 'mark-successful-boot', we enforce strict ordering
    between the two services to ensure 'prepare-boot' runs after
    'mark-successful-boot' is fully active/finished.

Testing done:
Verified that prepare-boot.service runs after mark-successful-boot.service

....
         Starting Call signpost to mark the …r all required targets are met....
         Starting Bottlerocket data store migrator...
[  OK  ] Started ACPI event daemon.
[  OK  ] Mounted Kernel Development Sources (Read-Only).
         Mounting Kernel Development Sources (Read-Write)...
[  OK  ] Mounted Kernel Development Sources (Read-Write).
[  OK  ] Finished Call signpost to mark the …ter all required targets are met..
         Starting Prepare Boot Directory (/boot)...
...

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.