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.

```diff
+#define STDIO_BAUDRATE      (115200U)
+#define STDIO_RX_BUFSIZE    (64U)
```