openssl: add libressl as a provider

At this point, libressl can be added to the openssl virtual package.

- Remove the entry package/libressl/Config.in from package/Config.in
- Remove the file: package/libressl/Config.in
- Add libressl entry to package/openssl/Config.in

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