export_report: use warn() to issue WARNING, so they go to stderr

Also count CONFIG_MODVERSIONS warnings, and print a NOTE at start of
SECTION 2 if any were issued.  Section 2 will be empty if the build is
lacking this CONFIG_ item, and user may have missed the warnings, as
they're off screen.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>