configs: remove CONFIG_SYS_MVFS

This was being used by some Marvell boards to enable some file system
related features (many of which have already been moved to Kconfig).
Make the future migration of the final 2 or 3 config options easier by
expanding #define CONFIG_SYS_MVFS into the options that it enables and
remove CONFIG_SYS_MVFS.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>