Add a reminder to disable DEBUG_INFO in 'make deb-pkg' examples

As mentioned in the "Disk space requirements" section, the
stock configuration for i386, amd64, and s390x produces an
absurdly large build tree and resulting kernel unless
CONFIG_DEBUG_INFO is disabled.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: Ben Hutchings <ben@decadent.org.uk>