insane.bbclass: fix new override syntax migration

A few variables and messages were not migrated over to the new override
syntax (_ to :).

Let's fix that.

Signed-off-by: Quentin Schulz <foss@0leil.net>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>