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/[e-mail]
Signed-off-by: Thomas Weißschuh <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

lib/Makefile

···
 CFLAGS_test_fpu_impl.o += $(CC_FLAGS_FPU)
 CFLAGS_REMOVE_test_fpu_impl.o += $(CC_FLAGS_NO_FPU)
 
-# Some KUnit files (hooks.o) need to be built-in even when KUnit is a module,
-# so we can't just use obj-$(CONFIG_KUNIT).
-ifdef CONFIG_KUNIT
 obj-y += kunit/
-endif
 
 ifeq ($(CONFIG_DEBUG_KOBJECT),y)
 CFLAGS_kobject.o += -DDEBUG

lib/kunit/Makefile

···
 endif
 
 # KUnit 'hooks' are built-in even when KUnit is built as a module.
-obj-y +=				hooks.o
+obj-$(if $(CONFIG_KUNIT),y) +=		hooks.o
 
 obj-$(CONFIG_KUNIT_TEST) +=		kunit-test.o
 obj-$(CONFIG_KUNIT_TEST) +=		platform-test.o