stmhal: Improve flash storage cache management.
Internal flash used for the filesystem is now written (from the cache) only after a 5s delay, or when a file is closed, or when the drive is unmounted from the host. This delay means that multiple writes can accumulate in the cache, and leads to less writes to the flash, making it last longer. It's implemented by a high-priority interrupt that takes care of flash erase and write, and flushing the cache. This is still only an interim solution for the flash filesystem. It eventually needs to be replaced with something that uses less RAM for the cache, something that can use more of the flash, and something that does proper wear levelling.