Fix uart_checker reading wrong characters

[lukamac]

Fixed

• fix wrong characters being printed due to sampling frequency misalignments. Increasing the clock frequency of the uart_checker by 4 instead of 2 fixed it. This consequently doubles the number of enqueued events.

Changed

• use enum for state holding instead of multiple bool flags
• use std::putc for printing instead of std::cout
• renamed rx_nb_bits to sent_bits and nb_bits to received_bits
• renamed stdout to print_stdout to not clash with the global stdout

Removed

• remove telnet since it was commented out anyway