boot/syslinux: prepare to install non-core images

Currently, we hard-code the path to the images we install, and
expect them to be from the core/ sub-dir.

Not all images we can install are located in core/. For example,
the efi boot images (to come later) are not located in core/.

Prepare the upcoming installation of extlinux by not expecting
images be in core/.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Lundquist <lists@zelow.no>
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>