tools: buildman: Don't use the working dir as build dir

When the U-Boot base directory happens to have the same name as the branch
that buildman is directed to use via the '-b' option and no output
directory is specified with '-o', buildman happily starts removing the
whole U-Boot sources eventually only stopped with the error message:

OSError: [Errno 20] Not a directory: '../<branch-name>/boards.cfg

Add a check to avoid this and also deal with the case where '-o' points
to the source directory, or any subdirectory of it.

Finally, tidy up the confusing logic for removing the old tree when using
-b. This is only done when building a branch.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Lothar Waßmann <LW@KARO-electronics.de>