Adds a basic support to add external platforms

[mdaffin]

fixes #41

Adds a new functions register_hardware_platform(). This functions takes a path to the hardware platform such as ${ARDUINO_SDK_PATH}/hardware/${PLATFORM} and creates the varibles:

${PLATFORM}_CORES_PATH if cores exists ${PLATFORM}_BOARDS_PATH if boards.txt exists ${PLATFORM}_BOARDS if boards.txt exists ${PLATFORM}_PROGRAMMERS_PATH if programmers exists ${PLATFORM}_VARIENTS if varients exists

${PLATFORM} is the uppercase of the last directory name and so is compatable with the previous variable names (ie hardware/arduino will create ARDUINO_CORES_PATHS...)

Removes the now unused load boards and programmers methods for the arduino platforms as this is now handeled by the new function.

Adds some new varibles; PLATFORMS lists all the registered platforms. VARIANTS lists all the registared variants. ${variant}.path the path of the pins header file for the variant this is now used instead as the include path rather then ${ARDUINO_VARIANTS_PATH}/${variant}

Modifies the print boards and programmers to list all the platform boards and programmers.

[queezythegreat]

WOW. You just made my day!

I haven't had any time to work on Arduino-CMake for the last couple of month and here I see someone has implemented the entire functionality. Sorry for not answering your pull request earlier.

I will try to review/integrate your patch by the end of this week.

[queezythegreat]

Hi, I have just integrated your feature into the develop branch. The documentation has been updated, and I fixed a minor bug relating to examples not being built properly. I want to thank for your great contribution, I did not need to change anything apart from the name of PLATFORMS (changed it to ARDUINO_PLATFORMS so that it is consistent with all the other variables).

Sorry again for keeping you hanging here :)

[queezythegreat]

I will close this issue for now, as I have merged in your changes. Reopen this issue if you want to further discuss my changes.