tools: remove old GDB scripts

Remove several old GDB Python scripts that are unused, and for the most
part, unusable, do to changes in the OS.

Change-Id: I3da820041cf741900bde83ca3a1c9a25c1f2b83a
tools: move GDB scripts to dedicated subdirectory

Change-Id: I7ae9f68db76d3e3458d9f6f5ab389ba5fb42bee4
tools: remove usart_stress

This tool, while perhaps useful, depends on an obscure module, and is
rarely used.

Change-Id: Ia4bad0b022db8e211fce79674b95be229427b26e