Provide the DIRVERSION as a constant

Provide the DIRVERSION as a constant

Depending on configure options, the version string used in the name
of the version specific directory differs. Provide this string also
as a constant, as it is needed for the upgrade procedure, in order to
be able to roll back.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Jose Lopes <jabolopes@google.com>