To actually get the filename in the error reporting struct instead of
the start of the path, let's use the MBED_FILENAME macro that MBED_ERROR
et al uses.

Mbed OS will soon be changing directory structure to the below:

```
[component name]
├── mbed_lib.json
├── CMakeList.txt
├── README.md
├── include
│   └── [component name]
│       └── internal
├── source
└── tests
    ├── <framework>
    │   └── <test suite one>
    ├── UNITTESTS
    │   └── <unit test suite one>
    └── TESTS
        └── <greentea test suite one>
```
This PR implements the above new directory structure for the current \platform directory.
**NOTE:** This PR does NOT include the refactoring of UNITTESTS as shown in the above directory structure. This will be done in a subsequent PR.