Polished gdb support, etc

Polished gdb support, etc

GDB handler re-done, removed the thread, removed the pauses,
Now as fast as possible for stepping trhu code.
Note you /need/ the dwarf-2 debug symbols for gdb to work
properly, a simple '-g' will not work.

Also added a mode that starts the gdb server and waits if the
AVR core detects a "crash". Added a piece if test unit to
test just that.

Signed-off-by: Michel Pollet <buserror@gmail.com>