openssl: introduce BR2_PACKAGE_OPENSSL_FORCE_LIBOPENSSL

Some packages that use openssl are not compatible with libressl, only
with the real openssl (known as libopenssl in Buildroot). So before we
add libressl as a provider for the openssl virtual package, we
introduce a BR2_PACKAGE_OPENSSL_FORCE_LIBOPENSSL hidden option that
packages incompatible with LibreSSL will be able to select.

This will allow packages that need OpenSSL to continue using "select",
without having to change to using "depends on" dependencies.

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