Update collectd, base of Luci statistics, to 5.4.2.

Patches have been refreshed.
905-fix-sigrok-upstream-patch-post-541 was removed as unnecessary.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

-update collectd to the current release 5.4.1
-fix sigrok glib check (upstream patch after 5.4.1)

Note:
Changes are required also to luci-statistics in Luci source.
Patch in http://patchwork.openwrt.org/patch/5303/

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>