gdbstub_read_wait
while ((ch = (gdbstub_read_wait())) != '$')
ch = gdbstub_read_wait();
xmitcsum = hex_to_bin(gdbstub_read_wait()) << 4;
xmitcsum += hex_to_bin(gdbstub_read_wait());
ch = gdbstub_read_wait();
ch = gdbstub_read_wait();