usbinstall: don't fail if a syslinux module is missing, to allow using both versions 4 and 5

src/usbinstall.sh

@@ -354,9 +354,11 @@ else
    cp $ISOMOUNTDIR/boot/isolinux/* boot/
    mv boot/isolinux.cfg boot/extlinux.conf
    sed -i "s|root=[^[:space:]]*|root=live:/openmamba_live/openmamba-live.iso|" boot/extlinux.conf
-   cp /usr/share/syslinux/{libcom32,libutil,vesamenu}.c32 boot/ || {
-      errorAndExit $"Error: can't copy some files to /boot directory"
-   }
+   for f in /usr/share/syslinux/{libcom32,libutil,vesamenu}.c32; do
+      if [ -r $f ]; then
+         cp $f boot/ || errorAndExit $"Error: can't copy bootloader file(s) to /boot directory"
+      fi
+   done
    rm -f boot/isolinux*
    umount $ISOMOUNTDIR
    rmdir $ISOMOUNTDIR