buflib: Add a define telling the per-alloc overhead.

This allows buflib clients to more accurately estimate the total memory usage.
It's still not 100% accurate because the handle table grows in blocks, thus
buflib might use more memory that caused by allocations directly.

Change-Id: I68338bb94f510ad188fcb588aebf895b5f9197c5

firmware/buflib.c

@@ -55,6 +55,8 @@
  * to find the start of the character array (and therefore the start of the
  * entire block) when only the handle or payload start is known.
  *
+ * UPDATE BUFLIB_ALLOC_OVERHEAD (buflib.h) WHEN THIS COOKIE CHANGES!
+ *
  * Example:
  * |<- alloc block #1 ->|<- unalloc block ->|<- alloc block #2      ->|<-handle table->|
  * |L|H|C|cccc|L2|crc|XXXXXX|-L|YYYYYYYYYYYYYYYY|L|H|C|cc|L2|crc|XXXXXXXXXXXXX|AAA|

firmware/include/buflib.h

@@ -53,6 +53,17 @@ struct buflib_context
     bool compact;
 };
 
+/**
+ * This declares the minimal overhead that is required per alloc. These
+ * are bytes that are allocated from the context's pool in addition
+ * to the actually requested number of bytes.
+ *
+ * The total number of bytes consumed by an allocation is
+ * BUFLIB_ALLOC_OVERHEAD + requested bytes + strlen(<name passed to
+ * buflib_alloc_ex()) + pad to pointer size
+ */
+#define BUFLIB_ALLOC_OVERHEAD (6*sizeof(union buflib_data))
+
 /**
  * Callbacks used by the buflib to inform allocation that compaction
  * is happening (before data is moved)