package/connman: clarify license of client code

While the core connman code is licensed under GPL-2.0, the client code is
GPL-2.0+ for compatibility with readline (which is GPL-3.0+).

Extend the _LICENSE with this info to clarify that linking against
readline is OK licensing wise.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>