Add shl_llog library logger

shl_llog is a small helper to make logging in libraries easier. It simply
forwards the messages to a configured helper function so users can
forward messages if required.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
shl_log: fix typo

Dlist -> Log copy-paste typo.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>