[configs/baseline] Use relative paths for syslinux like in releng profile

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>

configs/baseline/isolinux/isolinux.cfg

@@ -2,4 +2,4 @@ DEFAULT loadconfig
 
 LABEL loadconfig
   CONFIG /%INSTALL_DIR%/boot/syslinux/syslinux.cfg
-  APPEND /%INSTALL_DIR%/boot/syslinux/
+  APPEND /%INSTALL_DIR%/

configs/baseline/syslinux/syslinux.cfg

-UI menu.c32
+UI boot/syslinux/menu.c32
 MENU TITLE Arch Linux
 
 LABEL arch
 MENU LABEL Arch Linux
-LINUX /%INSTALL_DIR%/boot/%ARCH%/vmlinuz
-INITRD /%INSTALL_DIR%/boot/%ARCH%/archiso.img
+LINUX boot/%ARCH%/vmlinuz
+INITRD boot/%ARCH%/archiso.img
 APPEND archisobasedir=%INSTALL_DIR% archisolabel=%ARCHISO_LABEL%