fs: [WIP] Pull in open source FAT FS implementation

This implements the top layer of the FAT FS. It exports an user API
that would be abstacted by Zephyr FS API. It has another set of API
to interface with a bottom layer that would provide platform specific
file systems. A flash file system will be implemented for the bottom
layer.

Jira: ZEP-285
Origin: ELM Chan
URL: http://elm-chan.org/fsw/ff/00index_e.html


Change-Id: Id7c99dbdac9612b1a597bd8099b8a528824afae9
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>