[U-Boot] buildman fails if branch specifier contains a'/'

[Steve Rae]

example:

$ tools/buildman/buildman -b dev/to_denx broadcom
No section: 'make-flags'
boards.cfg is up to date. Nothing to do.
Building 1 commit for 4 boards (4 threads, 2 jobs per thread)
Traceback (most recent call last):
   File "tools/buildman/buildman", line 139, in <module>
     control.DoBuildman(options, args)
   File "/home/srae/work/to_denx/tools/buildman/control.py", line 221, 
in DoBuildman
     options.keep_outputs, options.verbose)
   File "/home/srae/work/to_denx/tools/buildman/builder.py", line 1040, 
in BuildBoards
     builderthread.Mkdir(self.base_dir)
   File "/home/srae/work/to_denx/tools/buildman/builderthread.py", line 
22, in Mkdir
     os.mkdir(dirname)
OSError: [Errno 2] No such file or directory: '../dev/to_denx'
$