package/easy-rsa: select the appropriate option for openssl binary

package/easy-rsa: select the appropriate option for openssl binary

Since openssl was converted to a virtual package,
BR2_PACKAGE_OPENSSL_BIN no longer exists: it was renamed to
BR2_PACKAGE_LIBOPENSSL_BIN, but easy-rsa was not changed accordingly.

easy-rsa needs to take into account the two providers of openssl, and
select the appropriate suboptions depending on which openssl
implementation was chosen.

Ideally, we would probably need a more elaborate option that ensures
easy-rsa doesn't have to know the details of which openssl
implementation is selected, but practically speaking with just two
providers of openssl at the moment, the proposed solution is good
enough.

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