openntpd: add optional dependency on LibreSSL

openntpd: add optional dependency on LibreSSL

OpenNTPD looks for a crypto library with tls_config_set_ca_mem, which
is available in LibreSSL but not OpenSSL. If tls_config_set_ca_mem is
found, crypto support is added to the build.

Because this is not currently checked, crypto support might not be
added due to the order of the build. Add a small check to see if
BR2_PACKAGE_LIBRESSL is selected, and if so, add a dependency for it.

Also add a note about this in the Config.in help section.

Signed-off-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>