openssl: new virtual package

To ease the transition to having both OpenSSL and LibreSSL, there has to be
a new virtual package introduced to handle both.

Instead of making a libssl, and adding OpenSSL and libressl to that package,
it will be far easier to move openssl to libopenssl and to make OpenSSL
a virtual package.  This offers a few advantages:

- BR2_PACKAGE_OPENSSL is still a visible symbol with no dependencies.
- It does not require a huge patch to convert every instance of
  OpenSSL -> libssl)
- Users will be able to update without ever having to select anything new.
- LibreSSL can be added at a later date to the virtual package.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
[Thomas: define BR2_PACKAGE_PROVIDES_HOST_OPENSSL to the value
"host-libopenssl" as we always want to use the original OpenSSL for
the host variant.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>