Feature/freertos linux port

[atigyi]

A FreeRTOS Linux simulator port implementation.

Contents:

• FreeRTOS posix BSP
• 2 new PRESETS: FREERTOS_LINUX_REL, FREERTOS_LINUX_DEV
• auto download of FreeRTOS kernel and FreeRTOS linux simulator into third_party dir
• example/freertos_linux which runs Embedded C Client from FreeRTOS kernel task
• Travis CI integration: builds FreeRTOS Linux port + build and run utests and itests

[DellaBitta]

I was able to run a test build and connect to the service with my gLinux machine. Nice Work!

My comment from this process is that example make file doesn't create a bin directory like the mqtt_logic_producer example does. I don't care which way we configure our builds, but I'd like for them to be consistent if possible.

Additionally make clean results in a bunch of errors on the freertosexample, too.

[atigyi]

Fixed the make clean by not emitting error upon non-existent files removal try.

The reason for freertos-linux example is not following the Embedded C Client's example directory structure is to be more familiar for FreeRTOS users. The example directory structure and build process follows the FreeRTOS example style. Additionally I think it would be even better to put this example into a FreeRTOS Plus package, maybe named: FreeRTOS Plus GCP IoT. Or like this: Linux_gcc_simple_tasks in the FreeRTOS Plus Linux Simulator package. All this in a mindset we don't want to host ports in our repo. Thus we shouldn't follow Embedded C Client style but the style of the port. Thus the FreeRTOS example should be similar to other FreeRTOS kernel / FreeRTOS Plus Examples.

[atigyi]

I moved the example a dir deeper to show what I mean. This way the example is almost identical to a FreeRTOS Plus Linux Simulator example.