[posix-sim] implement UART flush (#4072)
[posix-sim] implement UART flush (#4072)


srickardti/openthread,turon/openthread,lanyuwen/openthread,lanyuwen/openthread,openthread/openthread,chshu/openthread,chshu/openthread,jwhui/openthread,srickardti/openthread,lanyuwen/openthread,bukepo/openthread,jwhui/openthread,turon/openthread,srickardti/openthread,librasungirl/openthread,abtink/openthread,lanyuwen/openthread,bukepo/openthread,abtink/openthread,lanyuwen/openthread,bukepo/openthread,bukepo/openthread,jwhui/openthread,abtink/openthread,chshu/openthread,openthread/openthread,jwhui/openthread,chshu/openthread,abtink/openthread,openthread/openthread,librasungirl/openthread,turon/openthread,turon/openthread,srickardti/openthread,chshu/openthread,chshu/openthread,librasungirl/openthread,lanyuwen/openthread,librasungirl/openthread,openthread/openthread