Expose LQI from radio up through Thread stack. (#70)

* Add an LQI field to the radio packet structure.
* Add an LQI field to the ThreadMessageInfo.
* Add an LQI field to the scan result.
* Print LQI value in CLI scan result.

include/platform/radio.h

@@ -75,6 +75,8 @@ enum
 
     kPhyBitsPerOctet    = 8,
     kPhyUsPerSymbol     = ((kPhyBitsPerOctet / kPhySymbolsPerOctet) * 1000000) / kPhyBitRate,
+
+    kPhyNoLqi           = 0,       ///< LQI measurement not supported
 };
 
 /**
@@ -97,6 +99,7 @@ typedef struct RadioPacket
     uint8_t mPsdu[kMaxPHYPacketSize];  ///< The PSDU.
     uint8_t mChannel;                  ///< Channel used to transmit/receive the frame.
     int8_t  mPower;                    ///< Transmit/receive power in dBm.
+    uint8_t mLqi;                      ///< Link Quality Indicator for received frames.
     bool    mSecurityValid;            ///< Security Enabled flag is set and frame passes security checks.
 } RadioPacket;