openvpn: add option for --enable-password-save

Added configuration option BR2_PACKAGE_OPENVPN_PWSAVE that
adds --enable-password-save to OPENVPN_CONF_OPTS if selected.

[Thomas: rewrap Config.in help text, as suggested by Vicente.]

Signed-off-by: Andreas Wetzel <andreas.wetzel@nanotronic.ch>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>