package/connman : enable build without gnutls dependency

- add new configuration option '--enable-wispr' (disabled by default)
- move gnutls dependency to "wispr" config option

[Peter: drop gnutls ref for wchar, tweak help text, simplify .mk]
Signed-off-by: Heiko Abraham <abrahamh@web.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>