[build] update configure.ac for coverage flags
[build] update configure.ac for coverage flags

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