[configs/releng] Make syslinux.cfg (loadconfig) with relative paths.
We are only using these files only for SYSLINUX bootloader that supports ".." as directories. ISOLINUX jumps to archiso.cfg via isolinux.cfg PXELINUX goes directly to archiso.cfg via DHCP configuration. Suggested by Thomas. Signed-off-by: Gerardo Exequiel Pozzi <email@example.com>