Set correct path to OPENSSLCONFIG.

--- doc/mkcert.sh.orig	2012-01-31 15:25:06.611718182 +0100
+++ doc/mkcert.sh	2012-01-31 15:25:27.298222975 +0100
@@ -5,7 +5,7 @@
 
 OPENSSL=${OPENSSL-openssl}
 SSLDIR=${SSLDIR-/etc/ssl}
-OPENSSLCONFIG=${OPENSSLCONFIG-dovecot-openssl.cnf}
+OPENSSLCONFIG=${OPENSSLCONFIG-${SSLDIR}/dovecot-openssl.cnf}
 
 CERTDIR=$SSLDIR/certs
 KEYDIR=$SSLDIR/private
