fs: Adds diskio interface

Adds the diskio interface for the FAT file system.  This
revision uses RAM to emulate disk storage.

Origin: Original
Jira: ZEP-285
Change-Id: I7a30c8761d5ed9b564f1d1e08482c5ef199d7372
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>