mbed_rom_size values from mbed_app.json are ignored

**Describe the bug**

#270 has implemented mbed_rom_size support in targets.json file,
But it seems that if values is overwritten in the local mbed_app.json file, value is ignored

@wernerlewis 


**To Reproduce**
Steps to reproduce the behavior:

- choose a target with "mbed_rom_size" defined
- change the value in mbed_app.json: "target.mbed_rom_size"
- see in mbed_config.cmake

