tools: replace private DOCDIR with automake docdir

tools: replace private DOCDIR with automake docdir

Reuse automake variable instead of inventing a private variable.

The result of this command:
 git grep -wnl DOCDIR | xargs sed -i 's@DOCDIR@docdir@g'

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>