Use a specific hardware/package_index_bundled.json. AVR core version to 1.6.13

Previously, during the build, the full package_index.json was downloaded
and distributed with the Arduino IDE.
This lead to a situation where it was difficult to test new AVR cores
before publishing them to the public package_index.json.

Now the bundled AVR core is specificed in the file:
`hardware/package_index_bundled.json`
this index is loaded from the IDE at startup and the package_index.json
is overlayed on it.

This should also solve part of #5143 (Repeatable builds and snapshots of
package/library indexes)

Author: cmaglie

platform.txt

@@ -6,7 +6,7 @@
 # https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5-3rd-party-Hardware-specification
 
 name=Arduino AVR Boards
-version=1.6.12
+version=1.6.13
 
 # AVR compile variables
 # ---------------------