Bootloader overrides from config are not processed

### Description
mbed_app_start and friends are not processed by the config tool.

### Issue request type

- [ ] Enhancement
- [x] Bug

Start by tracing the config tool's handling of bootloader settings, using mbed_app_start and related overrides as the entry points. Confirm how these values are represented and where processing stops. Done means config-provided mbed_app_start and related settings are processed correctly; add or run focused tests if the existing test layout identifies a suitable location.

Written by the indexing model from the issue text.