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.
