x86, boot: Wrap debug printing in a new debug_putstr function

Change all instances of if (debug) putstr(...) to a new debug_putstr(...).
This allows a future change to conditionally stub out debug_putstr to save
space.

Signed-off-by: Joe Millenbach <jmillenbach@gmail.com>
Link: http://lkml.kernel.org/r/1342746282-28497-3-git-send-email-jmillenbach@gmail.com
Signed-off-by: Gokul Caushik <caushik1@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>