Remote FreeRTOS sources integration as git submodule.

[VincentDary]

Hello,

I suggest the following PR in order to use the official FreeRTOS source code as a git submodule rather than a partial copy of the source code in the FreeRTOS directory.

To test the PR, use the following command.

$ git clone -b remote-freeRTOS-integration  \
                   https://github.com/VincentDary/FreeRTOS-GCC-ARM926ejs.git \
                   --recurse-submodules
$ cd FreeRTOS-GCC-ARM926ejs && make