Add support for STM32F411E-DISCO board.

I was looking for why this board isn't supported in PlatformIO, and came across these issues: - platformio/platform-ststm32#51 - ARMmbed/mbed-os#2390 It appears from these discussions that there is no technical reason that the board isn't available and the process to add the board is documented. - https://os.mbed.com/platforms/ST-Discovery-F411E/ - https://os.mbed.com/teams/ST/wiki/steps-to-create-a-new-STM32-platform I am willing to go through the process to add this, as long as my effort would not be wasted. The concern I had is that there was [an announcement](https://os.mbed.com/blog/entry/Important-Update-on-Mbed/) that the Mbed OS is end-of-life, combined with this note on the ST wiki article above: > - The SDK code must be published on GitHub https://github.com/ARMmbed/mbed-os which leads me to wonder if my contribution would be accepted.