boot/syslinux: move comments out of define-block

The comments are not really part of the command we want to execute,
so move them out of the define-block. This also cleans up the output,
as they will no longer appear.

Reported-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>