rosclean: simplifying run-time dependencies

When roslaunch is started, it checks the file size of the local log
directory by calling:

    disk_usage = rosclean.get_disk_usage(d) [1]

The function `get_disk_usage` [2] in rosclean creates a subprocess and
calls `du -sb` on Linux systems (cf. [3]).

However, the `du` command, which busybox usually provides on an
embedded Linux image, does not support the `-b` option, and only the
`du` command from the coreutils [4] supports this option.
This issue was first reported in April 2013 on the meta-ros issue
tracker [5]. Hence, on the first iteration of this issue, the
commit db0c8d5c [6] simply adds the dependency on coreutils to
roslaunch.

However, this has certain disadvantages:
  - coreutils is licensed under GPLv3 and must not be deployed in a
    product, which is massively distributed to customers.
  - coreutils has larger file-system foot print than busybox and makes
    the minimal Linux images considerably larger.

As a fortuitous circumstance, Alexis Ballier [7] had already observed
this disadvantage and provides a patch [8, 9] that makes
`get_disk_usage` use `du -k`, which works with busybox and coreutils.

So, on the second iteration of this issue, this commit here patches
rosclean's `get_disk_usage` function with the aforementioned patch.
This slightly modifies the semantics of this function. However, I
believe this plays only a minor role for the overall intended
functionality to check if the log usage has reached a critical file-size
limit, and it allows us to remove the dependency on coreutils, which is
much more critical.

Andreas Baak reported the disadvantages of the previous solution, which
triggered the re-investigation. The current resolution has been
discussed and worked out in collaboration with Andreas Baak.

[1] https://github.com/ros/ros_comm/blob/9da29441f32575deb90ee73c9602a5527aacc966/tools/roslaunch/src/roslaunch/rlutil.py#L63
[2] https://github.com/ros/ros/blob/c6e91f9af1410ef183cfa273a7395f896d2d2a1f/tools/rosclean/src/rosclean/__init__.py#L120
[3] https://github.com/ros/ros/blob/c6e91f9af1410ef183cfa273a7395f896d2d2a1f/tools/rosclean/src/rosclean/__init__.py#L130
[4] http://cgit.openembedded.org/cgit.cgi/openembedded-core/tree/meta/recipes-core/coreutils/coreutils_8.23.bb?h=master
[5] https://github.com/bmwcarit/meta-ros/issues/60
[6] https://github.com/bmwcarit/meta-ros/commit/db0c8d5cd1700174888071a6f617d307a2e050e0
[7] https://github.com/aballier
[8] https://github.com/ros/ros/pull/76
[9] https://github.com/aballier/ros/commit/bbf1f945c7e3a54efca912d38fe8b1b2f5b63988



Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>
Signed-off-by: Andreas Baak <andreas.baak@bmw-carit.de>