test/py: put "Starting U-Boot" into separate log section

test/py: put "Starting U-Boot" into separate log section

The initial boot of U-Boot happens within the context of the first test
that needs to access the U-Boot console when there is no existing
connection. This keeps all activity nestled within test execution, which
fits well into the pytest model. However, this mingles the U-Boot startup
logs with the execution of some test(s), which hides find the boundary
between the two.

To solve this, wrap the "Starting U-Boot" logic into a separate log
section. If the user wishes, they can simply collapse this log section
when viewing the HTML log, to concentrate purely on the test's own
interaction.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>