subsys: disk: Refactor disk_access stuff into a directory

Disk IO functions are used by both FS and USB Mass Storage.
This patch refactors those from FS directory to a separate one.
In addition existing, config options were modified to make
stuff meaningful.

Jira: ZEP-1276

Change-Id: Ia2a2e18f3dbbbdb964c3dc0427d8138ad86134cd
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>