syslinux: add support for host-install

This makes it possible to build the syslinux/extlinux installer, so a
bootable USB drive, CompactFlash or SD card can be created by a user
script.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>