fs: add CONFIG_FILE_SYSTEM_LIB_LINK option

[JordanYates]

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, and the user is operating on the file system through the underlying library API anyway.

[JordanYates]

I think this should be outside of if FILE_SYSTEM, and/or named FILE_SYSTEM_RAW_API_ONLY. Rationale is that you may want, for some reason, to use both and the Kconfig name is confusing as it enables something while it actually removes VFS support of file systems and restricts access to raw api only.

Updated along these lines. Instead of an option that implies some new behaviour, add a symbol which links in the filesystem libraries which is selected by CONFIG_FILE_SYSTEM. Applications that don't want CONFIG_FILE_SYSTEM are free to enable CONFIG_FILE_SYSTEM_LIB_LINK themselves.

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