fs/romfs/Kconfig v3.0-rc7

ROMFS_FS

ROM file system support

This is a very small read-only file system mainly intended for
initial ram disks of installation disks, but it could be used for
other read-only media as well.  Read
<file:Documentation/filesystems/romfs.txt> for details.

To compile this file system support as a module, choose M here: the
module will be called romfs.  Note that the file system of your
root partition (the one containing the directory /) cannot be a
module.

If you don't know whether you need it, then you don't need it:
answer N.

ROMFS_FS

RomFS backing stores

Select the backing stores to be supported.

ROMFS_BACKED_BY_BLOCK

Block device-backed ROM file system support

This permits ROMFS to use block devices buffered through the page
cache as the medium from which to retrieve data.  It does not allow
direct mapping of the medium.

If unsure, answer Y.

ROMFS_BACKED_BY_MTD

MTD-backed ROM file system support

This permits ROMFS to use MTD based devices directly, without the
intercession of the block layer (which may have been disabled).  It
also allows direct mapping of MTD devices through romfs files under
NOMMU conditions if the underlying device is directly addressable by
the CPU.

If unsure, answer Y.