tools/mxsimage: Support building with LibreSSL

The mxsimage utility fails to compile against LibreSSL version < 2.7.0
because LibreSSL says it is OpenSSL 2.0, but it does not support the
complete OpenSSL 1.1 interface.

LibreSSL defines OPENSSL_VERSION_NUMBER with 0x20000000L and therefor
claims to have an API compatible with OpenSSL 2.0, but it implements
EVP_MD_CTX_new(), EVP_MD_CTX_free() and EVP_CIPHER_CTX_reset() only
starting with version 2.7.0, which is not yet released. OpenSSL
implements this function since version 1.1.0.

This commit will activate the compatibility code meant for
OpenSSL < 1.1.0 also for LibreSSL version < 2.7.0.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Jonathan Gray <jsg@jsg.id.au>