tools/hotplug: substitute XEN_SCRIPT_DIR on FreeBSD

Also remove DESTDIR from the path, this was most likely not intended.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
remove XEN_CONFIG_DIR from config

... because they are already in Paths.mk:
BINDIR, LIBEXEC, PRIVATE_BINDIR, PRIVATE_PREFIX, SBINDIR, SHAREDIR,
XEN_CONFIG_DIR, XEN_LOCK_DIR, XEN_PAGING_DIR, XEN_RUN_DIR. Remove unused
PKG_XEN_PREFIX, which was also incorrectly assigned to PRIVATE_PREFIX.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
rerun autogen.sh

Signed-off-by: Olaf Hering <olaf@aepfle.de>