subsys: fs: Add Virtual File system Switch (VFS) support

Add support for Virtual File system Switch (VFS) by
introducing mount point concept to Zephyr. This allows
the applications to mount multiple file systems at
different mount points (ex: "/fatfs" and "/nffs"). The
mount point structure contains all the necessary info
required to instantiate, mount and operate on file system.

Decouple applications from directly accessing individual
file systems API's or internal functions by introducing
file system registration mechanism in VFS.

Move the file system defination and mount responsibility
to application so that application can decide which file system
to use and where to mount.

Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>