buildman: Do not search under non-existing toolchain directories

The function glob.glob(pathname) is useful for expanding '*' in the
pathname. But it can be used even if '*' is not contained in the
pathname. In this case, glob.glob() returns an empty list if the given
pathname does not exist.

It means glob.glob is useful for all cases, not only for expanding '*'
but also for ignoring non-existing pathname.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
buildman: remove unnecessay sanity check

The confitonal statement "if config_fname:" is redundant
because ConfigParser.read() always succeeds even if an empty
string is given to its argument.

Besides, 'config_fname' always contains a non-empty string.
The variable 'config_fname', if empty, is set to '$HOME/.buildman'
at the previous line.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
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 in that case, 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>
buildman: improve readability of toolchain check