zabbix: preserve conffile over sysupgrade

zabbix: preserve conffile over sysupgrade

while doing a sysupgrade the conffile was lost because:
- the path in the Makefile is incorrect
- the parametrized /conffiles  define doesn't seem to work for me

So I added a /conffiles define for every zabbix component and corrected
the path. I think the path changed while upgrading to 2.0.0.

I also had an issue with "hostname" and "uname" not being present on my
system, so I added selects onto these busybox-applets if the native
tool is not installed. Zabbix could also call lsusb and lspci, but I
think it's okay if these calls don't work on an embedded device.

Additionally I deleted an old conffile "files/zabbix_agentd.conf"
which is unused since changeset 32497 (level up zabbix to version 2.0.0)

Signed-off-by: Stefan Hellermann <stefan@the2masters.de>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
SVN-Revision: 34491