src: uart_protocol: Use conditional (ENABLE_UART_PROTOCOL) to link only if used

This makes sure that functionality from the uart_protocol module is only used,
when ENABLE_UART_PROTOCOL is set. With the appropriate compiler/linker
settings, this module will be left out if ENABLE_UART_PROTOCOL is not set to 1.
TODO: Remove item regarding unused functions

This can also be achieved with the correct compiler and linker settings, which
makes it easier to maintain.