kunit: qemu_configs: SH: Respect kunit cmdline

kunit: qemu_configs: SH: Respect kunit cmdline

The default SH kunit configuration sets CONFIG_CMDLINE_OVERWRITE which
completely disregards the cmdline passed from the bootloader/QEMU in favor
of the builtin CONFIG_CMDLINE.
However the kunit tool needs to pass arguments to the in-kernel kunit core,
for filters and other runtime parameters.

Enable CONFIG_CMDLINE_EXTEND instead, so kunit arguments are respected.

Link: https://lore.kernel.org/r/20250407-kunit-sh-v1-1-f5432a54cf2f@linutronix.de
Fixes: 8110a3cab05e ("kunit: tool: Add support for SH under QEMU")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>