logmngr: add logread and default uci config
- all logging packages have a custom implmentation for logread, which is a command that can be invoked to view the logs. Hence, it makes sense for logmngr to have a logread to accompany it as well. Support for the same is added via this commit.
- the approach is simplistic and follows syslog-ng with the slight enhancement of reading the logfile from logmngr uci
- support to generate default uci config for logmngr which is to write logs to /var/log/messages