bazel: generate licenses.txt

Modify license_map rule to:

- Accept more than 1 targets
- Add '--asciidoctor' option to generate asciidoctor txt file
- Add support for multiple licenses for a single target (diffy_logo)

Also add test_license.sh for license_test rule to handle special
DO_NOT_DISTRIBUTE exceptions.

TESTED:
  bazel build Documentation:licenses.txt
  bazel test gerrit-pgm:pgm_license_test
  bazel test gerrit-gwtui:ui_module_license_test

Change-Id: Ic64b227fea34882721e6e064b1520cd9a4d5d4a4
diff --git a/tools/bzl/BUILD b/tools/bzl/BUILD
index 01ae92c..bfbbd21 100644
--- a/tools/bzl/BUILD
+++ b/tools/bzl/BUILD
@@ -1,4 +1,6 @@
 
 exports_files([
   "license-map.py",
-  "test_empty.sh"])
+  "test_empty.sh",
+  "test_license.sh",
+])