Add manual test case for AUM rollback

Currently there are several AUM tests that are being executed in LAVA that
ensures that the different scenarios are covered. Unfortunately due to the
nature of how LAVA works, these tests do not take into account an important
piece of software, the bootloader u-boot.

The bootloader is responsible of two important things to propoerly handle
the rollback:
- Increment the bootcount on each boot attempt
- Switch to an alternative boot method in case of failure

To ensure that the whole system is working add a new manual test case.

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