edk2: Move License.txt file to root

https://bugzilla.tianocore.org/show_bug.cgi?id=642

Add top level License.txt file with the BSD 2-Clause
License that is used by the majority of the EKD II open
source project content.  Merge copyright statements
from the BSD 2-Clause License files in each package
directory and remove the duplication License.txt
file from package directories.

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Andrew Fish <afish@apple.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>