The cu application page was an empty stub. Document the command:
description, options, escape sequences, configuration dependencies
and limitations.
Also add an rs485 configuration to the linum-stm32h753bi board with
the two on-board RS-485 transceivers (UART4 and USART6) and the cu
terminal enabled, plus a board documentation section showing how to
bridge the console to one of the RS-485 buses with cu. The RS-485
DE pins are handled automatically by the serial driver.
Tested on hardware.
Signed-off-by: Jorge Guzman <jorge.gzm@gmail.com>
