linux: drop the option to use the same version as that of the headers

It is no longer meaningful, now that we have the option to use the
kernel version for the linux headers, as it is more logical and more
versatile.

Add it to legacy.

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