uninstalled: Add an option to strip off the sysroot path

uninstalled: Add an option to strip off the sysroot path

As the data from meson is no longer relative path, it is not longer
possible to move gst-build around and run gst-uninstalled.py. This broke
cross-compilation usage, where you build on a host and run over NFS on
target. This adds an option to tell the script to strip off the host path
to the sysroot.