gstreamer1: don't rely on a recent coreutils for relative symlink support

Fixes:
http://autobuild.buildroot.net/results/3ae/3ae368cfd0ecd7be4c7e864590ec5dbf79d576ac/
http://autobuild.buildroot.net/results/4c9/4c951f31984e0eff21fc165cb68065f527c237e6/



And many more.

The recent change to create the legacy gstconfig.h symlink using ln -srf
unfortunately breaks on older distributions as it was only added in
coreutils 8.16 (2012).

Instead, "manually" create the relative symlink.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>