NcpSpi: Ensure spi tx frame preparation and callbacks happen from the same tasklet. (#455)

This commit changes the `NcpSpi` class implementation such that SPI
tx frame preparation always happens from the same task `mPrepareTxFrameTask`
This ensures that the callbacks to base class `Ncp` are also invoked from the
the same task (and not from the the task calling `OutboundFrameEnd()`).

examples/platforms/posix/spi-stubs.c

@@ -39,6 +39,9 @@ ThreadError otPlatSpiSlaveEnable(
     void *aContext
 )
 {
+    (void)aCallback;
+    (void)aContext;
+
     fprintf(stderr, "\nNo SPI support for posix platform.");
     exit(0);
 
@@ -57,6 +60,12 @@ ThreadError otPlatSpiSlavePrepareTransaction(
     bool aRequestTransactionFlag
 )
 {
+    (void)anOutputBuf;
+    (void)anOutputBufLen;
+    (void)anInputBuf;
+    (void)anInputBufLen;
+    (void)aRequestTransactionFlag;
+
     return kThreadError_NotImplemented;
 }