maven_jar() Add define_license() rule

To define custom licenses from standalone build mode, define_license()
rule is needed.

Change-Id: Icff6dbb85fc0b49a7d9b1afed2c93c7857094411

maven_jar.bucklet

diff --git a/maven_jar.bucklet b/maven_jar.bucklet
index 81a4ff1..bf25d6e 100644
--- a/maven_jar.bucklet
+++ b/maven_jar.bucklet
@@ -48,6 +48,16 @@
 MAVEN_CENTRAL = 'MAVEN_CENTRAL:'
 MAVEN_LOCAL = 'MAVEN_LOCAL:'
 
+def define_license(name):
+  n = 'LICENSE-' + name
+  genrule(
+    name = n,
+    cmd = 'ln -s $SRCS $OUT',
+    srcs = [n],
+    out = n,
+    visibility = ['PUBLIC'],
+  )
+
 def maven_jar(
     name,
     id,