base.bbclass changes broke BitBake 1.8.19

[Denys Dmytriyenko <denis@denix.org>]

Chris,

The c67b95955d023c1f198531b4f06100a40b05efa7 has introduced the following 
problem with BitBake 1.8.19 and even af81d52e48cb6d4f91fff0c70e09dfd421e0057d 
didn't fix it. Any thoughts? Thanks.

Traceback (most recent call last):
  File "/OE/bitbake/bin/bitbake", line 143, in <module>
    main()
  File "/OE/bitbake/bin/bitbake", line 140, in main
    cooker.cook()
  File "/OE/bitbake/lib/bb/cooker.py", line 640, in cook
Parsing .bb files, please wait...done.
NOTE: Parsing finished. 0 cached, 7152 parsed, 358 skipped, 945 masked.
    return self.buildTargets(pkgs_to_build)
  File "/OE/bitbake/lib/bb/cooker.py", line 527, in buildTargets
NOTE: build ${DISTRO_NAME} ${DISTRO_VERSION}: started
    bb.event.fire(bb.event.BuildStarted(buildname, targets, 
self.configuration.event_data))
  File "/OE/bitbake/lib/bb/event.py", line 67, in fire
    if tmpHandler(event) == Handled:
  File "tmpHandler(e)", line 10, in tmpHandler
  File "/OE/bitbake/lib/bb/__init__.py", line 100, in plain
    bb.msg.warn(''.join(args))
TypeError: warn() takes at least 2 arguments (1 given)

-- 
Denys