BMessage: add AddFlat taking a const BFlattenable

It seems the const was forgotten in the BeOS version. But there is no
reason not to have it.
*actually* update libsdl_x86

I failed to rebuild the package in the previous try...

Fixes #11429.
SerialConnect: add scripting support

It's now possible to configure the serial port through scripting:
hey SerialConnect set databits to 8
hey SerialConnect set parity to even
etc.

More useful is the ability to connect and disconnect from the port:
hey SerialConnect get port # returns "usb0"
hey SerialConnect set port to usb0
hey SerialConnect delete port

This allows a script to automatically disconnect SerialConnect, do
something with the serial port, and then reconnect SerialConnect. This
can be used for example to run a bootloader and update a firmware
through the same serial port used for debugging, something that's not
easily possible on other systems.