Configure additional classpath through sbt.extraClasspath

Modify the default launcher configuration for sbt so extra classpath
entries for sbt can be configured through `sbt.extraClasspath` system
property.

launch/src/main/input_resources/sbt/sbt.boot.properties

@@ -8,6 +8,7 @@
   class: ${sbt.main.class-sbt.xMain}
   components: xsbti,extra
   cross-versioned: ${sbt.cross.versioned-false}
+  resources: ${sbt.extraClasspath-}
 
 [repositories]
   local