Support building for multiple platforms with Ceedling.

The plugin behaves very similar to the project:<name> option, but allows the extra project files to be inside some folder that can be specified, keeping your project root clean of .yml files if you are handling a lot of them.

Create a folder in your machine for Ceedling plugins if you do not have one already. e.g. ~/some/place/for/plugins:

$ mkdir -p ~/some/place/for/plugins

cd into the plugins folder and clone this repo:

$ cd ~/some/place/for/plugins
$ git clone https://github.com/deltalejo/platforms-ceedling-plugin.git platforms

Add the plugins path to your project.yml to tell Ceedling where to find them if you have not done it yet. Then add platforms plugin to the enabled plugins list:

:plugins:
  :load_paths:
    - ~/some/place/for/plugins
  :enabled:
    - platforms

Add the folder(s) which contains (or will contain) the extra project files to your project.yml:

:project:
  :platforms_paths:
    - platforms

Put the extra project files inside the specified folder:

.
├── platforms
│   ├── platform-1.yml
│   ├── platform-2.yml
│   └── platform-3.yml
├── src
├── test
└── project.yml

Call Ceedling specifying the desired platform. e.g.:

$ ceedling platform:platform-1 test:all