Avoid use of non-portable echo -n in grub-mkconfig

util/grub-mkconfig_lib.in(version_test_gt): "echo -n" is not really needed,
final newline is stripped by command substitution.

util/grub.d/10_kfreebsd.in, util/grub.d/10_linux.in,
util/grub.d/20_linux_xen.in: change how list is built, to avoid echo -n
completely.

util/grub.d/30_os-prober.in: add spaces to printed line directly

Closes 43668.

util/grub.d/10_kfreebsd.in

@@ -143,9 +143,12 @@ EOF
 EOF
 }
 
-list=`for i in /boot/kfreebsd-* /boot/kernel/kernel ; do
-        if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
-      done`
+list=
+for i in /boot/kfreebsd-* /boot/kernel/kernel ; do
+  if grub_file_is_not_garbage "$i" ; then
+    list="$list $i"
+  fi
+done
 prepare_boot_cache=
 boot_device_id=
 title_correction_code=