test/py: Make the number of SPL banners seen a variable

Currently we have the option to tell the console code that we should
ignore the SPL banner. We also have an option to say that we can see it
a second time, and ignore it. However, some platforms such as TI AM64x
will have us see the SPL banner three times. Rather than add an
"spl3_skipped" option, rework the code. By default we expect to see the
banner once, but boards can specify seeing it as many times as they
expect to.

Signed-off-by: Tom Rini <trini@konsulko.com>