Validate configs_nuttx with updated LiteXCITest.

[enjoy-digital]

And update the notes.

Also, is it really required to use 1Mbps UART? Cause it's hardcoded in NuttX?

[enjoy-digital]

@trabucayre: I don't seem to be able to build the config_nuttx:

florent@panda:~/dev/litex/litex_hw_ci/nuttx$ ./make.py --soc-json=../build_arty_vexriscv-secure-1-core/soc.json --nuttx-clean --nuttx-build

                                    __   _ __      _  __
                                   / /  (_) /____ | |/_/
                                  / /__/ / __/ -_)>  <
                                 /____/_/\__/\__/_/|_|
                              LiteX Hardware CI/Linux Tests.

                      Copyright 2024 / Enjoy-Digital and LiteX developers.

Makefile:22: chip/Make.defs: No such file or directory
make[1]: *** No rule to make target 'chip/Make.defs'.  Stop.
make: *** [tools/Unix.mk:758: arch/risc-v/src_clean] Error 2

[trabucayre]

1Mbps seems required. With 115200 prompt is more or less working (Byte lost).

I have added a git clean to avoid situation where nuttx is not fully configured/unconfigured. kconfig-frontends is required (README.md updated)