Config.in.legacy: add separator to description for users

When legacy symbols are selected, the menuconfig shows the explanation for
users, immediately followed by the line "Legacy options removed in 2013.08".
In order to visually keep these apart, we introduce a separator.

Note that a line with all spaces is not correctly shown by menuconfig (the
width will not be kept at 80 characters, so the output is '***   ***').
A dashed line seemed a logical alternative, and with another dashed line on
top, the description becomes a block.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>