Ncp: Update the ncp implementation to use new buffering model (#329)

This commit modifies the `NcpBase` and `NcpUart` classes to use new
Ncp buffer model (defined `in NcpFrameBuffer` class).

Here are the main changes in this commit:

- It changes the 'NcpUart` to adopt the `NcpFrameBuffer` for storing the
  outbound frames. This allows multiple frames to be queued for tx.

- `NcpUart` is changed so that the outgoing frames are encoded and
  sent over to Uart in smaller chunks. This helps reduce the required
  buffer size for storing the encoded outbound uart data.

- The spinel command handler methods in `NcpBase` are modified to return
  ThreadError status to indicate success or failure (e.g., when there is no
  buffer space for a response frame).

- If the response to a spinel command cannot be sent (no buffer space) the
  `NcpBase` remembers the set of TIDs (transaction ids) of such commands
  so to reply later with error status `SPINEL_STATUS_NOMEM` when buffer
  space becomes available. This ensures that spinel commands will get
  a response even in case of full buffer.

- The `NcpSpi` class is modified to address the changes from `NcpBase` but
  this commit does not change the buffer model for the `NcpSpi`. It adds
  `spi-stubs.c` to enable building the ncp app with spi feature under
  posix platform.

examples/platforms/posix/Makefile.am

@@ -46,6 +46,12 @@ libopenthread_posix_a_SOURCES             = \
     uart.c                                  \
     $(NULL)
 
+if OPENTHREAD_ENABLE_NCP_SPI
+libopenthread_posix_a_SOURCES           += \
+    spi-stubs.c                            \
+    $(NULL)
+endif
+
 noinst_HEADERS                            = \
     platform-posix.h                        \
     $(NULL)

examples/platforms/posix/spi-stubs.c

@@ -0,0 +1,73 @@
+/*
+ *  Copyright (c) 2016, Nest Labs, Inc.
+ *  All rights reserved.
+ *
+ *  Redistribution and use in source and binary forms, with or without
+ *  modification, are permitted provided that the following conditions are met:
+ *  1. Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *  2. Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ *  3. Neither the name of the copyright holder nor the
+ *     names of its contributors may be used to endorse or promote products
+ *     derived from this software without specific prior written permission.
+ *
+ *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ *  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ *  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ *  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ *  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ *  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ *  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ *  POSSIBILITY OF SUCH DAMAGE.
+ */
+
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <platform/uart.h>
+#include <platform/spi-slave.h>
+
+// Spi-slave stubs
+
+ThreadError otPlatSpiSlaveEnable(
+    otPlatSpiSlaveTransactionCompleteCallback aCallback,
+    void *aContext
+)
+{
+    fprintf(stderr, "\nNo SPI support for posix platform.");
+    exit(0);
+
+    return kThreadError_NotImplemented;
+}
+
+void otPlatSpiSlaveDisable(void)
+{
+}
+
+ThreadError otPlatSpiSlavePrepareTransaction(
+    uint8_t *anOutputBuf,
+    uint16_t anOutputBufLen,
+    uint8_t *anInputBuf,
+    uint16_t anInputBufLen,
+    bool aRequestTransactionFlag
+)
+{
+    return kThreadError_NotImplemented;
+}
+
+// Uart
+
+void otPlatUartSendDone(void)
+{
+}
+
+void otPlatUartReceived(const uint8_t *aBuf, uint16_t aBufLength)
+{
+    (void)aBuf;
+    (void)aBufLength;
+}