all: passwd group hosts ethers networks rpc services protocols \
        .... ypservers

Try add the following to your yp Makefile

ypservers.time: $(DIR)/ypservers
        @(sed -e '/^#/d' -e s/#.*$$// -e 's/[ ][ ]*$$//' \
            -e '/\\$$/s/\\$$/ /' $(DIR)/ypservers $(CHKPIPE))\
        |( awk 'BEGIN { OFS="\t"; } $$1 !~ /^#/ {print $$0, $$0}' $(CHKPIPE))\
        |( sed -e 's/[ ][ ]*/ /g' $(CHKPIPE))\
        | $(MAKEDBM) - $(YPDBDIR)/$(DOM)/ypservers;
        @touch ypservers.time;
        @echo "updated ypservers";
        @if [ ! $(NOPUSH) ]; then $(YPPUSH) -d $(DOM) ypservers; fi
        @if [ ! $(NOPUSH) ]; then echo "pushed ypservers"; fi

ypservers: ypservers.time