MAINTAINERS: handle buildsystem

MAINTAINERS: handle buildsystem

Tweaking configure and friends is most likely not of much interest for
anyone beside the tools maintainers.

List such files, which are currently covered by "THE REST", in the
TOOLSTACK section. Also update list of stubdom related files.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>