kunit: Always descend into kunit directory during build

For kbuild to properly clean up these build artifacts in the subdirectory,
even after CONFIG_KUNIT changed do disabled, the directory needs to be
processed always.

Pushing the special logic for hook.o into the kunit Makefile also makes the
logic easier to understand.

Link: https://lore.kernel.org/r/20250813-kunit-always-descend-v1-1-7bbd387ff13b@linutronix.de


Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>