Reset by default LD_LIBRARY_PATH in xds-project.conf

Reset by default LD_LIBRARY_PATH in xds-project.conf

New environment-setup-*-agl-linux file, used to set environment
variables to use an SDK, now checks if LD_LIBRARY_PATH is set or not.
And not empty LD_LIBRARY_PATH is considered as misconfigured and
prevent to use SDK. Following is the error message displayed in that
case :

Your environment is misconfigured, you probably need to
'unset LD_LIBRARY_PATH' but please check why this was set in the
first place and that it's safe to unset.
The SDK will not operate correctly in most cases when LD_LIBRARY_PATH
is set.
For more references see:
  http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html#AEN80
  http://xahlee.info/UnixResource_dir/_/ldpath.html

Change-Id: I5f89f68d95a78e9b73f89e487ec74dd314bd5c48
Signed-off-by: Sebastien Douheret <sebastien.douheret@iot.bzh>