[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 <vmlinuz386@yahoo.com.ar>

configs/releng/syslinux.dual/syslinux.cfg

 DEFAULT loadconfig
 
 LABEL loadconfig
-  CONFIG /%INSTALL_DIR%/boot/syslinux/archiso.cfg
-  APPEND /%INSTALL_DIR%/
+  CONFIG archiso.cfg
+  APPEND ../../

configs/releng/syslinux/syslinux.cfg

 DEFAULT loadconfig
 
 LABEL loadconfig
-  CONFIG /%INSTALL_DIR%/boot/syslinux/archiso.cfg
-  APPEND /%INSTALL_DIR%/
+  CONFIG archiso.cfg
+  APPEND ../../