Introduce NCP related configuration options (#983)

This commits add a set of `OPENTHREAD_CONFIG_NCP` configuration
parameters to specify size of different NCP related buffers (for
both `NcpUart` and `NcpSpi`).

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

@@ -372,4 +372,44 @@
 #define OPENTHREAD_CONFIG_NUM_SLAAC_ADDRESSES                   4
 #endif  // OPENTHREAD_CONFIG_NUM_SLAAC_ADDRESSES
 
+/**
+ * @def OPENTHREAD_CONFIG_NCP_TX_BUFFER_SIZE
+ *
+ *  The size of NCP message buffer in bytes
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_NCP_TX_BUFFER_SIZE
+#define OPENTHREAD_CONFIG_NCP_TX_BUFFER_SIZE                   512
+#endif  // OPENTHREAD_CONFIG_NCP_TX_BUFFER_SIZE
+
+/**
+ * @def OPENTHREAD_CONFIG_NCP_UART_TX_CHUNK_SIZE
+ *
+ *  The size of NCP UART TX chunk in bytes
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_NCP_UART_TX_CHUNK_SIZE
+#define OPENTHREAD_CONFIG_NCP_UART_TX_CHUNK_SIZE               128
+#endif  // OPENTHREAD_CONFIG_NCP_UART_TX_CHUNK_SIZE
+
+/**
+ * @def OPENTHREAD_CONFIG_NCP_UART_RX_BUFFER_SIZE
+ *
+ *  The size of NCP UART RX buffer in bytes
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_NCP_UART_RX_BUFFER_SIZE
+#define OPENTHREAD_CONFIG_NCP_UART_RX_BUFFER_SIZE              1500
+#endif  // OPENTHREAD_CONFIG_NCP_UART_RX_BUFFER_SIZE
+
+/**
+ * @def OPENTHREAD_CONFIG_NCP_SPI_BUFFER_SIZE
+ *
+ *  The size of NCP SPI (RX/TX) buffer in bytes
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_NCP_SPI_BUFFER_SIZE
+#define OPENTHREAD_CONFIG_NCP_SPI_BUFFER_SIZE                  1500
+#endif  // OPENTHREAD_CONFIG_NCP_SPI_BUFFER_SIZE
+
 #endif  // OPENTHREAD_CORE_DEFAULT_CONFIG_H_

src/ncp/ncp_spi.hpp

@@ -33,6 +33,12 @@
 #ifndef NCP_SPI_HPP_
 #define NCP_SPI_HPP_
 
+#ifdef OPENTHREAD_CONFIG_FILE
+#include OPENTHREAD_CONFIG_FILE
+#else
+#include <openthread-config.h>
+#endif
+
 #include <ncp/ncp_base.hpp>
 #include <ncp/ncp_buffer.hpp>
 
@@ -61,9 +67,10 @@ public:
 private:
     enum
     {
-        kSpiBufferSize   = 1500, // Spi buffer size (should be large enough to fit a max length frame + spi header).
-        kTxBufferSize    = 512,  // Tx Buffer size (used by mTxFrameBuffer).
-        kSpiHeaderLength = 5,    // Size of spi header.
+        kSpiBufferSize   = OPENTHREAD_CONFIG_NCP_SPI_BUFFER_SIZE, // Spi buffer size (should be large enough to fit a
+                                                                  // max length frame + spi header).
+        kTxBufferSize    = OPENTHREAD_CONFIG_NCP_TX_BUFFER_SIZE,  // Tx Buffer size (used by mTxFrameBuffer).
+        kSpiHeaderLength = 5,                                     // Size of spi header.
     };
 
     uint16_t OutboundFrameSize(void);

src/ncp/ncp_uart.hpp

@@ -33,6 +33,12 @@
 #ifndef NCP_UART_HPP_
 #define NCP_UART_HPP_
 
+#ifdef OPENTHREAD_CONFIG_FILE
+#include OPENTHREAD_CONFIG_FILE
+#else
+#include <openthread-config.h>
+#endif
+
 #include <ncp/ncp_base.hpp>
 #include <ncp/flen.hpp>
 #include <ncp/hdlc.hpp>
@@ -118,9 +124,10 @@ private:
 
     enum
     {
-        kUartTxBufferSize = 128,  // Uart tx buffer size.
-        kTxBufferSize = 512,      // Tx Buffer size (used by mTxFrameBuffer).
-        kRxBufferSize = 1500,     // Rx buffer size (should be large enough to fit one whole (decoded) received frame).
+        kUartTxBufferSize = OPENTHREAD_CONFIG_NCP_UART_TX_CHUNK_SIZE,  // Uart tx buffer size.
+        kTxBufferSize = OPENTHREAD_CONFIG_NCP_TX_BUFFER_SIZE,          // Tx Buffer size (used by mTxFrameBuffer).
+        kRxBufferSize = OPENTHREAD_CONFIG_NCP_UART_RX_BUFFER_SIZE,     // Rx buffer size (should be large enough to fit
+                                                                       // one whole (decoded) received frame).
     };
 
     enum UartTxState