McuSupportPlugin: explicit set up of FreeRtos

Currently:

• most of the information for creating the McuPackage objects for BoardSDK and FreeRTOS is hardcoded and happens by itself within the plugin, based mostly on pattern recognition of some environment variable.

Should be:

• Each one of them is explicitly described in the JSON file (as in, the specific information needed to build it is provided there)
• Also explicitly created in the plugin with that information. Not relying on the presence or absence of a given environment variable (which by the way, is gone anyway)