Curently, we have a choice to select between stripping and not
stripping. This is legacy code from back when we had a third option,
sstrip (super-strip).

Since we removed sstrip, stripping or not stripping is now just a
boolean rather than a choice.

Make it so.

We make BR2_STRIP_strip default to 'y' to keep the current behaviour of
defaulting to stripping.

Move BR2_STIP_none to legacy, and instruct the user to review the new
setting.

Drop any reference to BR2_STRIP_none in comments.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>