libressl: add a recipe to support openssh

libressl: add a recipe to support openssh

After reading through this:

https://github.com/openssh/openssh-portable/pull/48

and this thread:

https://lists.mindrot.org/pipermail/openssh-unix-dev/2017-October/036344.html

I've concluded that this is the best of the three not-great options. The alternatives:

- bundle libressl inside openssh packages
- keep openssh dependent on openssl 1.0 and wait until upstream does something

are both inferior. Libressl is used with openssh in OpenBSD and in OS X,
so it did get at least some testing in the real world.

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>