Control system timer allocation via configurator

[forGGe]

System timer allocation was implemented and made consistent in #274 .
Goal of this task is to add possibility to control system timer via JSON configurator.

Additional field can be placed in the platform subobject like that:

{
  "platform": {
    "use_systmr": true
  }
}

Platforms to extend:

• stm32
• tm4c
• particle electron