modules: littlefs copy the naive crc function from LittleFS

LittleFS requires a `lfs_crc()` function, however the basic crc
function provided by LittleFS gets disabled when a custom configuration
header is used.

To ensure identical crc behavior let Zephyr have a copy of the basic
lfs_crc.

This allows to ensure identical behavior when moving glue code into
Zephyr repo, and allow us to change to an existing crc implementation
provided natively by Zephyr at a later time.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>