src: uart_protocol.c: Add functions to output messages from program space
src: uart_protocol.c: Flush output before putting something into buffer

This makes sure that the transmitting buffer is flushed, so the whole
transmitting buffer is completely available for new content.
src: Implement module for dynamic logging functionality
src: uart_protocol: Rename function and command for retrieving brightness
src: uart.h: Increase outgoing buffer from 64 to 128
src: uart_protocol: Implement functions for handling dynamic logging facility
src: uart_protocol.c: uart_protocol_commands: Add TODO item
doc: UART_PROTOCOL.md: Add documentation about logging functions
README.md: Add section about new logging facility
src: ldr: Make use of dynamic logging facility
src: brightness: Make use of dynamic logging facility
src: main.c: Make use of dynamic logging facility

This also introduces an option to enable logging globally during
initialization.
src: uart_protocol: Make use of dynamic logging facility
src: datetime: Make use of dynamic logging facility
src: user: Make use of dynamic logging facility for IR commands