Updated external library paths to be ignored by git.

Updated external library paths to be ignored by git.

The downloaded cereal library is now placed in extlibs/cereal/cereal.
Updated the '.gitignore' file, accordingly.  Without this change in
place, changes to extlibs/cereal/SConscript would be incorrectly
ignored.

Lastly, ignore all downloaded and unpacked expat library files and
directories.

Change-Id: Ic2a211bd421d25e92449427c64d096e785d6282e
Signed-off-by: Ossama Othman <ossama.othman@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/457
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Erich Keane <erich.keane@intel.com>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>