You need to extract boundary relations data before telling mkgmap to process it.
Use osmconvert and osmfilter to do that. Then use mkgmap's built in boundary processor to process it. Lastly use the bounds argument when compiling.
osmconvert xxx.osm.pbf --out-o5m >xxx.o5m
osmfilter xxx.o5m --keep-nodes= --keep-ways-relations="boundary=administrative =postal_code postal_code=" --out-o5m > xxx-boundaries.o5m
java -cp mkgmap.jar uk.me.parabola.mkgmap.reader.osm.boundary.BoundaryPreprocessor xxx-boundaries.o5m boundary