Add heap poisoning mechanism to fill absorbed block header with the right pattern.

Note: block_absorb() is called in tlsf_free(), which is eventually called in multi_heap_free()
after the memory fill is done. As the block_absorb merges 2 blocks together, the previous block
header of the merged block is now in the middle of the memory block but not filled with 0xfe.

- Remove dependencies with the heap component of the IDF.
- The tlsf_poison_fill_region_hook() function is defined as weak in IDF and checked for NULL
in block_absorb() function in order to minimize the dependencies between IDF and the TLSF.
- The the implementation of tlsf_poison_fill_region_hook() must be provided at the discretion
of the user.

tlsf.c

@@ -456,6 +456,12 @@ static inline __attribute__((always_inline)) block_header_t* block_absorb(block_
 	/* Note: Leaves flags untouched. */
 	prev->size += block_size(block) + block_header_overhead;
 	block_link_next(prev);
+
+	if (block_absorb_post_hook != NULL)
+	{
+		block_absorb_post_hook(block, sizeof(block_header_t), POISONING_AFTER_FREE);
+	}
+
 	return prev;
 }