* Add otPlatRadioGetReceiveSensitivity() API
* Add a new otPlatRadioGetReceiveSensitivity() API to get sensitivity value;
* Use the receive sensitivity value as the noise floor for link metric computation;
* Remove some unused noise floor related functions.
* Add SPINEL_PROP_PHY_RX_SENSITIVITY Spinel/NCP property
This commit adds a new class `LinkQualityInfo` which stores all
relevant information about quality of a link, including average
received signal strength (RSS), link margin, and link quality value.
The RSS average is obtained using an adaptive exponential moving
average filter. A unit-test is also added for the new class.
The Neighbor class now includes an instance of LinkQualityInfo. The
RSS average is updated from 'mac.cpp' whenever a valid/verified packet
is received from the corresponding neighbor. The mesh_forwarder,
mle_router, and mle modules are modified to use the new methods from
the newly added class.
This change addresses issue #175.
Jonathan Hui