[radio] add field to radio packet enable/disable CCA (#2344)

Defining `OPENTHREAD_CONFIG_DISABLE_CCA_ON_LAST_ATTEMPT` to use this
feature.

include/openthread/platform/radio.h

@@ -107,6 +107,7 @@ typedef struct otRadioFrame
     bool     mSecurityValid: 1; ///< Security Enabled flag is set and frame passes security checks.
     bool     mDidTX: 1;         ///< Set to true if this frame sent from the radio. Ignored by radio driver.
     bool     mIsARetx: 1;       ///< Set to true if this frame is a retransmission. Should be ignored by radio driver.
+    bool     mIsCcaEnabled: 1;  ///< Set to true if CCA must be enabled for this packet. False otherwise.
 
     /**
      * The timestamp when the frame was received (milliseconds).

src/core/mac/mac.cpp

@@ -868,6 +868,19 @@ void Mac::HandleBeginTransmit(void)
     Frame &sendFrame(*mTxFrame);
     otError error = OT_ERROR_NONE;
 
+#if OPENTHREAD_CONFIG_DISABLE_CCA_ON_LAST_ATTEMPT
+
+    // Disable CCA for the last attempt
+    if (mTransmitAttempts == (sendFrame.GetMaxTxAttempts() - 1))
+    {
+        sendFrame.mIsCcaEnabled = false;
+    }
+    else
+#endif
+    {
+        sendFrame.mIsCcaEnabled = true;
+    }
+
     if (mCsmaAttempts == 0 && mTransmitAttempts == 0)
     {
         sendFrame.SetPower(mMaxTransmitPower);

src/core/openthread-core-default-config.h

@@ -1008,4 +1008,14 @@
 #define OPENTHREAD_CONFIG_ENABLE_DYNAMIC_MPL_INTERVAL           0
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_DISABLE_CCA_ON_LAST_ATTEMPT
+ *
+ * Define as 1 to disable CCA on the last transmit attempt
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_DISABLE_CCA_ON_LAST_ATTEMPT
+#define OPENTHREAD_CONFIG_DISABLE_CCA_ON_LAST_ATTEMPT           0
+#endif
+
 #endif  // OPENTHREAD_CORE_DEFAULT_CONFIG_H_