buildman: fix to display warning message for missing [toolchain] section

buildman: fix to display warning message for missing [toolchain] section

Toolchains.__init__ is expected to display a warning message
when the [toolchain] section is missing from ~/.buildman file.
But it never works.
In that case, instead, buildmain fails with an error message
which is difficult to understand:

  Traceback (most recent call last):
    File "tools/buildman/buildman", line 126, in <module>
      control.DoBuildman(options, args)
    File "/home/foo/u-boot/tools/buildman/control.py", line 78, in DoBuildman
      toolchains = toolchain.Toolchains()
    File "/home/foo/u-boot/tools/buildman/toolchain.py", line 106, in __init__
    config_fname)
  NameError: global name 'config_fname' is not defined

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>