subsys: fs: Add Flash Circular Buffer

Zephyr already supports NFFS as a storage layer, but it might
be a little bit too heavyweight for certain applications in
memory-restricted ICs.

This module is response for need of Lightweight flash storage
capability. FCB is ported form MyNewt as native zephyr module.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>