I have occasionally found that I have killed off gdb, and had no way to 
recover a debug session to QEMU.  Also the detach/kill sequence does not 
work correctly protocol wise in the QEMU gdb-stub.  This patch addresses 
these problems.

I implemented the serial protocol commands the same way as in KGDB.

? = Query state, but also clear the breakpoints.
      - KGDB/gdb do the same thing so that in case you lose your
        session or context, gdb always send the "?" command during the
        connect sequence.  This ensures that gdb has a clean slate
        for breakpoints and run control.

D = Detach and clear all breakpoints with return "OK"

k = Do the same thing as D for now
      In the future this can be used to kill the target
      emulation.  But for now it makes gdb and other gdb serial debugger
      happy.

signed-off-by: jason.wessel@windriver.com

Thanks,
Jason.