I know this feature has been added again, but its accessible only through OSGi console. I developed a simple web interface to change log levels at runtime.
I uploaded the project on my github page. It compiles with Maven 3.0.5 as deployment package, you can install it on any running kura instance.
I find it very useful when developing/debugging, hope it may help someone else.
Yes, kura already supports it from OSGi console. Connect to the console with telnet <host> 5002, commands are getkuraloglevel
setkuraloglevel <loggername> <level>
where level is one of TRACE, DEBUG, INFO, WARN, CRITICAL, ERROR.
Those commands are defined in kura/org.eclipse.kura.core.log/src/main/java/org/eclipse/kura/core/log/LoggerCommandProvider.java