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/core_alloc.c

@@ -96,3 +96,10 @@ void core_print_block_at(int block_num, char* buf, size_t bufsize)
 {
     buflib_print_block_at(&core_ctx, block_num, buf, bufsize);
 }
+
+#ifdef DEBUG
+void core_check_valid(void)
+{
+    buflib_check_valid(&core_ctx);
+}
+#endif