tools/hotplug: use XEN_SCRIPT_DIR instead of hardcoded path

tools/hotplug: use XEN_SCRIPT_DIR instead of hardcoded path

Helper scripts get installed into XEN_SCRIPT_DIR, but initscripts,
helper scripts and udev rules still refer to the hardcoded location
/etc/xen/scripts/. Update scripts, rules and Makefile to refer to
@XEN_SCRIPT_DIR@ instead.

Update configure.ac to substitute the path in files using
XEN_SCRIPT_DIR. Remove XEN_SCRIPT_DIR from StdGNU.mk and SunOS.mk, its
already in Paths.mk.

Note: In SunOS.mk it refered to PRIVATE_PREFIX, which was introduced by
9f6d07bac9f14aaa8bafe38b4a217e61a3a57842 ("tools: get rid of hardcoded
config dirs").

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
[ ijc -- ran autogen.sh as requested ]