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/include/crc32.h

@@ -18,10 +18,12 @@
  * KIND, either express or implied.
  *
  ****************************************************************************/
+#include <stdint.h>
+
 #ifndef _CRC32_H
 #define _CRC32_H
 
-unsigned crc_32(const void *src, unsigned len, unsigned crc32);
+uint32_t crc_32(const void *src, uint32_t len, uint32_t crc32);
 
 #endif