[mle] fix bug in MLE attach behavior (#3850)
[mle] fix bug in MLE attach behavior (#3850)

Bug was introdcued in 5a7faa5.

A device performing MLE attach first sends an MLE Parent Request looking
for routers. If no MLE Parent Response is received from a neighboring router
where the link quality is 3 in both directions, the device shall send a
second MLE Parent Request that includes both routers and REEDs.

This commit adds the bi-directional link quality check to determine whether
a second MLE Parent Request should be sent.
chshu/openthread,bukepo/openthread,lanyuwen/openthread,lanyuwen/openthread,abtink/openthread,openthread/openthread,chshu/openthread,chshu/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread,turon/openthread,bukepo/openthread,lanyuwen/openthread,lanyuwen/openthread,abtink/openthread,chshu/openthread,abtink/openthread,lanyuwen/openthread,srickardti/openthread,bukepo/openthread,lanyuwen/openthread,bukepo/openthread,abtink/openthread,jwhui/openthread,turon/openthread,librasungirl/openthread,turon/openthread,chshu/openthread,openthread/openthread,srickardti/openthread,librasungirl/openthread,librasungirl/openthread,chshu/openthread,srickardti/openthread,turon/openthread,librasungirl/openthread,openthread/openthread,jwhui/openthread