Allocate another 1K bytes to CC2538 stack (#654)

Reduce the size of message buffer to save 1k bytes SRAM space for stack.

examples/platforms/cc2538/startup-gcc.c

@@ -59,7 +59,7 @@ extern void RFCoreRxTxIntHandler(void);
 extern void RFCoreErrIntHandler(void);
 extern void main(void);
 
-static uint64_t stack[384] __attribute__((section(".stack")));
+static uint64_t stack[512] __attribute__((section(".stack")));
 
 __attribute__((section(".vectors"), used))
 void (*const vectors[])(void) =

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

@@ -42,7 +42,7 @@
  *
  */
 #ifndef OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS
-#define OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS               48
+#define OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS               40
 #endif  // OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS
 
 /**