package/qt5/qt5base: add BR2_PACKAGE_QT5BASE_OPENSSL

Commit c110e48c disabled openssl support
on QT 5.6, this has the side-effect of breaking the build of
python-pyqt5 because support of openssl is enabled on python-pyqt5 if
BR2_PACKAGE_OPENSSL is selected

To fix this issue, add a new BR2_PACKAGE_QT5BASE_OPENSSL option in
qt5base and use it in python-pyqt5

Fixes:
 - http://autobuild.buildroot.org/results/e92991308d47649cecc4084e41ab5711ec96831f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Peter: make _OPENSSL a blind option, add libressl logic for 5.6.x]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>