legacy: i386 is legacy, inform the user

When we dropped support for i386, we added the corresponding opiton to
the legacy menu, but forgot to select BR2_LEGACY.

Fix that.

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