make TARGET=mytarget DEFINES=MYLOG, MYSIZE=100, MYTRACE
The build system does however _NOT_ take care to rebuild things if the DEFINES change so most likely a 'make clean' is advisable. To ease usage the DEFINES may be saved individually per target with the goal 'savedefines' similiar to savinf the target.
The 6502-based target leverage the DEFINES mechanism by introducing so-called 'high level config macros' which allow to configure Contiki per target AND per project.
Most of the time there's exactly one reasonable set of high level config macros for every combination of target and project. Therefore it makes sense to place them into CVS.