buflib: Add crc field protecting buflib cookie integrity

This should catch the case of buffer misuse which results
in corrupted cookie of next allocation. The check is performed
on move_block() so it may be a bit late.

There is buflib_check_valid() provided which checks the
integrity of all cookies for given context.
On DEBUG build with --sdl-thread this check is carried out
for core_ctx on every context switch to catch problems earlier.

Change-Id: I999d4576084592394e3dbd3bdf0f32935ff5f601
Reviewed-on: http://gerrit.rockbox.org/711
Reviewed-by: Thomas Martitz <kugel@rockbox.org>

firmware/target/hosted/sdl/thread-sdl.c

@@ -32,6 +32,7 @@
 #include "kernel.h"
 #include "thread.h"
 #include "debug.h"
+#include "core_alloc.h"
 
 /* Define this as 1 to show informational messages that are not errors. */
 #define THREAD_SDL_DEBUGF_ENABLED 0
@@ -382,6 +383,9 @@ void switch_thread(void)
         } /* STATE_SLEEPING: */
     }
 
+#ifdef DEBUG
+    core_check_valid();
+#endif
     cores[CURRENT_CORE].running = current;
 
     if (threads_status != THREADS_RUN)