fs: Make API naming adhere to the appropriate namespace

The namespace allocated for the filesystem API is fs_* and FS_*. That
means all symbols and defines should adhere to it.

Jira: ZEP-1155

Change-Id: I422310448b7c7c347f621aea6d7b1d97ef25c94d
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>