boot/syslinux: add option to install the EFI image

syslinux can now also build an EFI application.

If the target is 64-bit, we build the 64-bit EFI app,
otherwise we build the 32-bit EFI app.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Frank Hunleth <fhunleth@troodon-software.com>
Tested-by: Frank Hunleth <fhunleth@troodon-software.com>
Tested-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>