make possible enable pool, but disabled in system heap.
also compatible for further extra heap use/not use pool.
Now use MM_HEAP_MEMPOOL_THRESHOLD -1 to indicate pool disabled.
Signed-off-by: buxiasen <buxiasen@xiaomi.com>
There are many small memory block in NuttX system, eg: struct tcb_s,
struct inode, etc, and several disadvantages about them:
1.Their frequent allocate and free cause the system memory fragmentation.
2.Since each memory block has an overhead, the utilization of small memory
blocks is relatively low, which will cause memory waste.
So we can use mempool to alloc smallo block, to improve alloc speed
and utilization, to reduce fragmentation.
Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>
