boards: Initial support for Ellduino

Hello

This is a first version of support for ELL-i's Ellduino development board.

Short description of the board can be found here:
https://github.com/Ell-i/Hackathon/wiki/Ellduino-development-board

The board does not have an external oscillator and therefore had to move stm32f0 HSE clock initialization to f0 discovery board initialization.

you could use `CLOCK_CORECLOCK` here