openldap: upgrade to 2.4.20

1. upgrade to 2.4.20
2. remove two backup patches
2. integrate two patches to fix CVE-2015-1545 and CVE-2015-1546
3. disable bdb/hdb backend, since BerkeleyDB 6.0.20+ license is
   incompatible with LDAP

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>