test/py: fix CONFIG_SPL test

The Python ini file parser that's used to parse .config converts all keys
to lower-case. Hence, all queries against the results must use lower-case.
Fix u_boot_console.ensure_spawned() to test CONFIG_SPL correctly, or the
connection will fail for boards that have SPL.

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