openssl: unconditionally define BR2_PACKAGE_PROVIDES_HOST_OPENSSL

Fixes:
http://autobuild.buildroot.net/results/207/207d0ca1fe5328e675246c851fcb0d5685f8c0bd/
http://autobuild.buildroot.net/results/546/546a56d6dd39e5e9ecfe25fd36a00510e6c0c45b/



host-openssl may be used without openssl being enabled for the target, so
move BR2_PACKAGE_PROVIDES_HOST_OPENSSL outside the BR2_PACKAGE_OPENSSL
conditional.

While we're at it, add a comment explaining what this magic config symbol does.

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