Update logic protecting lookup of syslinux's source in tools/boot/wheezy/boot-x86
Steve pointed out that this lookup is only here so that helper scripts
can grab those sources and make them available in a public location
such as http://cdimage.debian.org/cdimage/cd-sources/. The sources are
not embedded in the generated ISO and are thus not strictly needed
for building the ISO image.
This should happen even when we're not building source ISOs but it's
not needed if we don't have any similar policy in place. Thus CONF.sh
is now leaving $ARCHIVE_EXTRACTED_SOURCES unset by default and the
source will only be looked up if that value is set.