mbed compile --flash does not respect 'artifact_name'

When using `mbed compile --flash`, it uses the default binary name for flashing (the parent directory) instead of the one given by the mbed_app.json parameter `artifact_name`. It correctly generates a binary with the artifact_name during linking.

mbed --version: 1.10.5

`mbed_app.json`: `{ artifact_name: "bar", ... }`

`mbed compile --flash` output:
```
Building project bar (UBLOX_EVK_ODIN_W2, GCC_ARM)
Scan: foo-cwd
Link: bar
Elf2Bin: bar
...
Image: ./BUILD/UBLOX_EVK_ODIN_W2/GCC_ARM/bar.bin
[mbed] ERROR: Build program file (firmware) not found "./BUILD/UBLOX_EVK_ODIN_W2/GCC_ARM/foo-cwd.bin"
```

Where foo-cwd is the parent directory name and the default name, and bar is the name given by artifact_name in mbed_app.json.