[cli] make CLI buffer sizes configurable (#2351)

* [core] Make CLI buffer sizes configurable.

At the moment, we allocate 512 bytes for a receive buffer and 1kB for
transmit, which while it doesn't seem a lot, is in fact a big commitment
for a CLI interface.  In cases where the platform does its own
buffering, this can be significantly reduced.

These add definitions that mirror the current settings so they may be
changed.

* [cli] Define UART buffers according to config.

Use the definitions produced in the previous commit to set the buffer
sizes.

* [core, cli] Rename OPENTHREAD_CONFIG_CLI_LINE_LENGTH

to `OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH` as per review in pull request 2351.

src/cli/cli_uart.hpp

@@ -111,9 +111,9 @@ public:
 private:
     enum
     {
-        kRxBufferSize = 512,
-        kTxBufferSize = 1024,
-        kMaxLineLength = 128,
+        kRxBufferSize = OPENTHREAD_CONFIG_CLI_UART_RX_BUFFER_SIZE,
+        kTxBufferSize = OPENTHREAD_CONFIG_CLI_UART_TX_BUFFER_SIZE,
+        kMaxLineLength = OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH,
     };
 
     otError ProcessCommand(void);

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

@@ -231,6 +231,36 @@
 #define OPENTHREAD_CONFIG_ADDRESS_QUERY_MAX_RETRY_DELAY         28800
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH
+ *
+ *  The maximum size of the CLI line in bytes
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH
+#define OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH                   128
+#endif
+
+/**
+ * @def OPENTHREAD_CONFIG_CLI_UART_RX_BUFFER_SIZE
+ *
+ *  The size of CLI UART RX buffer in bytes
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_CLI_UART_RX_BUFFER_SIZE
+#define OPENTHREAD_CONFIG_CLI_UART_RX_BUFFER_SIZE               512
+#endif
+
+/**
+ * @def OPENTHREAD_CONFIG_CLI_TX_BUFFER_SIZE
+ *
+ *  The size of CLI message buffer in bytes
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_CLI_UART_TX_BUFFER_SIZE
+#define OPENTHREAD_CONFIG_CLI_UART_TX_BUFFER_SIZE               1024
+#endif
+
 /**
  * @def OPENTHREAD_CONFIG_MAX_CHILDREN
  *