Using external cores with the 1.6.7 architecture

After introducing the 1.6.7 version of the compiler, we can no longer change the include paths used during the compilation because the Arduino-builder uses predefined include paths and tools for each architecture and each variant.
In order for the Arduino-builder to access any core files other than the default ones, the extra files need to be placed in the v167/hardware directory.
At the time of writing this document, three external cores have been used with the 1.6.7 compiler, each of which has to be added in a specific directory. Below you can see the desired structure as it's used in development version of the compiler, used for compiling chibitronics, microchip chipKit and esp8266 projects.

/opt/codebender/arduino-core-files/v167/hardware$ tree -L 4 chibitronics/
chibitronics/
└── hardware
    └── esplanade
        └── 1.6.0
            ├── boards.txt
            ├── cores
            ├── platform.txt
            ├── programmers.txt
            └── variants

5 directories, 3 files
/opt/codebender/arduino-core-files/v167/hardware$ tree -L 2 chipKIT
chipKIT
└── pic32
    ├── boards.txt
    ├── bootloaders.txt
    ├── compiler
    ├── cores
    ├── libraries
    ├── platformManual.txt
    ├── platforms.txt
    ├── platform.txt
    ├── platformURL.txt
    ├── tools
    ├── unsupported_boards.txt
    └── variants

6 directories, 7 files
/opt/codebender/arduino-core-files/v167/hardware$ tree -L 2 esp8266/
esp8266/
└── esp8266
    ├── boards.txt
    ├── bootloaders
    ├── cores
    ├── doc
    ├── ISSUE_TEMPLATE.md
    ├── libraries
    ├── LICENSE
    ├── package
    ├── platform.txt
    ├── programmers.txt
    ├── README.md
    ├── tests
    ├── tools
    └── variants

9 directories, 6 files