consider this situation: no more fresh block, meantime an item with such a big size come in, and currently no block inuse has sufficient space to save this item, the code will run into the modified line, and KV_MGR_WORKSPACE will be set to KV_BLK_INVALID. this will cause no matter what size of an item wanna come in again, kv_blk_search_suitable just return KV_BLK_INVALID because "if (KV_MGR_WORKSPACE == KV_BLK_INVALID)" is true.
1. a true wear-leveling kv fs for norflash, especially optimize for some onchip norflash with "write once on one single write unit" like stm32l4, a true "no earse before write" flash algorithm.
2. an "as less as possible" gc strategy, do best to save norflash's life.
3. full "power down protection" support
4. see "examples" of kv, project in "TencentOS_tiny_EVB_MX_Plus", with onchip flash and qspiflash sample.
