kconfig: refactor conf_touch_dep()

If this function fails to touch a dummy header due to missing parent
directory, then it creates it and touches the file again.

This was needed because CONFIG_FOO_BAR was previously tracked by
include/config/foo/bar.h. (include/config/foo/ may not exist here)

This is no longer the case since commit 0e0345b7 ("kbuild: redo
fake deps at include/config/*.h"); now all the fake headers are placed
right under include/config/, like include/config/FOO_BAR.

Do not try to create parent directory, include/config/, which already
exists.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>