config-utils.mk: add `addslash` and `rwildcard` functions
config-utils.mk: add `addslash` and `rwildcard` functions

Normalize directory names with `addslash` and
do recursive directory globbing via `rwildcard`.

Signed-off-by: Tim Janik <6ec6ff229c4e1c5bbd5933ee92e6c5832ff023bb@gnu.org>

tim-janik/beast,swesterfeld/beast,swesterfeld/beast,tim-janik/beast,swesterfeld/beast,tim-janik/beast,tim-janik/beast,tim-janik/beast,tim-janik/beast,swesterfeld/beast,tim-janik/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast