Internal flash VFS writes too slow

Copying python_payload (60 files, 312kb or 624kb) over usb disk mode takes ~40 seconds.

The initial tarball extract takes quite a while too, but not that bad.

q3k suggests that _flash_write could use a block cache https://git.card10.badge.events.ccc.de/badge23/firmware/-/blob/08b05dab5fa53651f32c26abab99ab7ce4a1a5dc/recovery/main/rec_vfs.c#L22