ubox: add log_trailer_null option to uci

this allow us to use syslog tcp with \0 trailer
instead of \n trailer (logread -0 option)

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45329 3c298f89-4303-0410-b956-a3cf2f4a3e73