image_ipkg: remove defaults

Empty strings are the default anyways and the ipkg index is rarely needed.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
sysstat: change suffix to tar.bz2 (tar.gz not available anymore)
libmikmod: change URL to SF mirror
nickel: change URL to SF mirror
sjinn: change URL to SF mirror
splashutils: change URL to SF mirror

Old URL not working anymore, berlios stopped operations earlier this
year, but luckily SF.net imported berlios projects so we can download it
there. However package still fails to build. ;-)
opencv: update download url

Old SF download urls on http://opencv.org/downloads.html ar broken,
files are not there anymore. According to
http://answers.opencv.org/question/41024 you can download those from
GitHub.