Arduino 1.5.x - support new library layout and plaform specific system libs

- Add support for 1.5.x library layout.
  (https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification)

- Allow using Arduino 1.5.x platform specific system libraries.
  Arduino 1.5.x has additional directories for platform (vendor/architecture)
  specific libraries - look there when searching for libraries.