fs: Add a sample app to demo Zephyr file system API

Demonstrates the Zephyr file system APIs. This currently
runs on FAT FS using RAM emulation of disk.

Origin: Original
Jira: ZEP-285 ZEP-228 ZEP-481 ZEP-446
Change-Id: If459b8c2f819a2a38c8c4d805ec3f689dfcdef17
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>