Backport v2023 <- v2024dev3: Improve pre conditions secure boot

Backport !549 (merged)

Improve the wording of the pre conditions section in the secure boot test to instruct the flashing to the correct u-boot image based on the release under test.

infrastructure/apertis-issues#350 (closed)

Signed-off-by: Walter Lozano walter.lozano@collabora.com