fs: add `FILE_SYSTEM_LIB_LINK` option

Add an option that links in the underlying file system libraries as
usual, but doesn't compile in the Zephyr file system abstraction layer.

This can be useful to avoid linking in the entire filesystem
implementation through the `fs_file_system_t` API struct if only a
subset of the functions are used.

Signed-off-by: Jordan Yates <jordan@embeint.com>