libssh2: make crypto library selectable

Add PACKAGECONFs 'openssl' and 'gcrypt' to make crypto library
selectable. Only one of them could be set. If they are both set,
'gcrypt' is invalide.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>