boot/syslinux: make the sub-options a choice

Currently it is possible to choose either, both or none of
the pxelinux or isolinux images.

But it does not make sense to build none or both, as we need
at least one to boot the target, and the target can not use
more than one.

So, we need one and only one image to be selected at once.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Lundquist <thomasez@redpill-linpro.com>
Cc: Frank Hunleth <fhunleth@troodon-software.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Tested-by: Frank Hunleth <fhunleth@troodon-software.com>
Tested-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>