[mle] only handle MLE attach messages when router role is enabled (#2252)
[mle] only handle MLE attach messages when router role is enabled (#2252)


erja-gp/openthread,turon/openthread,jwhui/openthread,LeZhang2016/openthread,openthread/openthread,mszczodrak/openthread,LeZhang2016/openthread,srickardti/openthread,georgecpr/openthread,pvanhorn/openthread,aeliot/openthread,LeZhang2016/openthread,aeliot/openthread,srickardti/openthread,bukepo/openthread,jwhui/openthread,xiaom-GitHub/openthread,gandreello/openthread,lanyuwen/openthread,bukepo/openthread,turon/openthread,chshu/openthread,pvanhorn/openthread,xiaom-GitHub/openthread,mszczodrak/openthread,pvanhorn/openthread,gandreello/openthread,librasungirl/openthread,bukepo/openthread,turon/openthread,georgecpr/openthread,LeZhang2016/openthread,aeliot/openthread,LeZhang2016/openthread,abtink/openthread,pvanhorn/openthread,georgecpr/openthread,aeliot/openthread,mszczodrak/openthread,georgecpr/openthread,openthread/openthread,fbsder/openthread,jwhui/openthread,gandreello/openthread,xiaom-GitHub/openthread,fbsder/openthread,librasungirl/openthread,georgecpr/openthread,pvanhorn/openthread,erja-gp/openthread,lanyuwen/openthread,openthread/openthread,fbsder/openthread,chshu/openthread,LeZhang2016/openthread,erja-gp/openthread,srickardti/openthread,erja-gp/openthread,abtink/openthread,lanyuwen/openthread,lanyuwen/openthread,abtink/openthread,xiaom-GitHub/openthread,lanyuwen/openthread,openthread/openthread,abtink/openthread,chshu/openthread,lanyuwen/openthread,mszczodrak/openthread,mszczodrak/openthread,chshu/openthread,gandreello/openthread,chshu/openthread,librasungirl/openthread,bukepo/openthread,jwhui/openthread,erja-gp/openthread,chshu/openthread,mszczodrak/openthread,fbsder/openthread,xiaom-GitHub/openthread,georgecpr/openthread,librasungirl/openthread,erja-gp/openthread,xiaom-GitHub/openthread,fbsder/openthread,pvanhorn/openthread,gandreello/openthread,aeliot/openthread,fbsder/openthread,turon/openthread,srickardti/openthread,aeliot/openthread,gandreello/openthread