Intel Hex flashing time increase in latest micro:bit builds

Intel Hex flashing time has increased in the main branch with armcc compared with the micro:bit factory release from commit 1436bdcc67029fdfc0ff03b73e12045bb6a9f272.

The measurements below have been done in macOS 11 with rsync and this Intel Hex file: [ihex-v2-upy.hex.zip](https://github.com/ARMmbed/DAPLink/files/7218655/ihex-v2-upy.hex.zip)
For comparison I've also added a snapshot of the `develop` branch with armcc and GCC.

| Build | Flashing time (s) |
|--------------------------|-------------------|
| armc 1436bdc (255) | ~13.5 |
| armcc main 8509fea | ~16.6 |
| armcc develop 930ef1df | ~16.5 |
| GCC develop 930ef1df | ~17.5 |

I've only run this on a micro:bit V2, so it's possible this might affect other ports as well, but I haven't tested it.