Kernel: Add print functionality for the kernel

[thomasw04]

A function for printing formatted strings should be added.

This function should select an underlying concept for performing the print.
For now, this should be either UART or ARM Semihosting. Per default UART should be selected, Semihosting should be configurable at compile-time.

This is blocked by OsirisRTOS/stm32l4-hal#6