Add soc_term.py

Adds soc_term.py, a direct replacement for the soc_term tool originally
written in C [1]. This Python version is slightly simpler and does not
need to be compiled (obviously). It is therefore reasonable to have it
here in build.git for use on QEMU and QEMUv8. We can later get rid of
the soc_term project in the manifest file of those platforms.

This script is also useful with FVP, although fvp.xml does not use it;
in a custom work flow the 'xterm' command is replaced by a script which
connects the FVP telnet ports to soc_term.py via the socat command.

Link: [1] https://github.com/linaro-swg/soc_term


Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>