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

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.