license check: recognize bsd 2 clause

third_party/libaom uses a BSD 2 clause license:
This source code is subject to the terms of the BSD 2 Clause License

R=thestig@chromium.org

Change-Id: I7b1dda5d5c485353b8024386541b853d12f0299c
Reviewed-on: https://chromium-review.googlesource.com/827543Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/master@{#524391}

third_party/devscripts/licensecheck.pl

@@ -538,6 +538,8 @@ sub parselicense($) {
         $license = "BSD-like $license";
     } elsif ($licensetext =~ /GOVERNED BY A BSD-STYLE SOURCE LICENSE/) {
         $license = "BSD-like $license";
+    } elsif ($licensetext =~ /BSD 2 Clause License/) {
+        $license = "BSD (2 clause) $license";
     } elsif ($licensetext =~ /BSD 3-Clause license/) {
         $license = "BSD (3 clause) $license";
     }