core: doc: do not exclude CPU specific values

This patch ensures that doxygen will add the CPU dependent defines for stack sizes.

core/include/thread.h

@@ -102,13 +102,16 @@ struct _thread {
 #endif
 };
 
- /**
+/**
  * @def THREAD_STACKSIZE_DEFAULT
  * @brief A reasonable default stack size that will suffice most smaller tasks
  */
 #ifndef THREAD_STACKSIZE_DEFAULT
 #error THREAD_STACKSIZE_DEFAULT must be defined per CPU
 #endif
+#ifdef DOXYGEN
+#define THREAD_STACKSIZE_DEFAULT
+#endif
 
 /**
  * @def THREAD_STACKSIZE_IDLE
@@ -117,6 +120,9 @@ struct _thread {
 #ifndef THREAD_STACKSIZE_IDLE
 #error THREAD_STACKSIZE_IDLE must be defined per CPU
 #endif
+#ifdef DOXYGEN
+#define THREAD_STACKSIZE_IDLE
+#endif
 
 /**
  * @def THREAD_EXTRA_STACKSIZE_PRINTF
@@ -126,6 +132,9 @@ struct _thread {
 #ifndef THREAD_EXTRA_STACKSIZE_PRINTF
 #error THREAD_EXTRA_STACKSIZE_PRINTF must be defined per CPU
 #endif
+#ifdef DOXYGEN
+#define THREAD_EXTRA_STACKSIZE_PRINTF
+#endif
 
 /**
  * @def THREAD_STACKSIZE_MAIN