Configure: Redo the logic for finding build file templates

Build file templates would be looked up like this if the user gave us
an additional directory to look for configuration files and build file
templates:

    $OPENSSL_LOCAL_CONFIG_DIR/$OSTYPE-Makefile.tmpl
    $SOURCEDIR/Configurations/$OSTYPE-Makefile.tmpl
    $OPENSSL_LOCAL_CONFIG_DIR/Makefile.tmpl
    $SOURCEDIR/Configurations/Makefile.tmpl

So for example, if the user created his own Makefile.tmpl and tried to
use it with a unixly config, it would never be user because we have a
unix-Makefile.tmpl in our Configurations directory.  This is clearly
wrong, and this change makes it look in this order instead:

    $OPENSSL_LOCAL_CONFIG_DIR/$OSTYPE-Makefile.tmpl
    $OPENSSL_LOCAL_CONFIG_DIR/Makefile.tmpl
    $SOURCEDIR/Configurations/$OSTYPE-Makefile.tmpl
    $SOURCEDIR/Configurations/Makefile.tmpl

Reviewed-by: Rich Salz <rsalz@openssl.org>