fs: jffs2: Move SYS_JFFS2_SORT_FRAGMENTS to Kconfig

Move the symbol SYS_JFFS2_SORT_FRAGMENTS to Kconfig and use the only
remaining part of doc/README.JFFS2 that is still relevant and useful to
the help for this option.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>