utils/get-developers: make it callable from elsewhere than the toplevel directory

utils/get-developers: make it callable from elsewhere than the toplevel directory

get-developers tries to open DEVELOPERS in the current directory, so it
breaks when calling it from elsewhere than the toplevel Buildroot directory.

Traceback (most recent call last):
  File "../utils/get-developers", line 107, in <module>
    __main__()
  File "../utils/get-developers", line 26, in __main__
    devs = getdeveloperlib.parse_developers(os.path.dirname()
  File "/home/peko/source/buildroot/utils/getdeveloperlib.py", line 161, in parse_developers
    with open(os.path.join(basepath, "DEVELOPERS"), "r") as f:
IOError: [Errno 2] No such file or directory: '/home/peko/source/buildroot/output-foo/DEVELOPERS'

Fix it by instead figuring out where the DEVELOPERS file is relative to the
location of get-developers (E.G. one level up).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[Arnout:
  - add realpath to support a symlinked get-developers script;
  - pass devs_dir argument to check_developers() to support -c in subdir;
  - convert basepath to absolute path to support -f option.
]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>