STDIN switch during input

[sy2002]

Make IO$GETCHAR smarter, so that switching STDIN does not require "one last keypress" on the old STDIN any more before reading from the new STDIN

[sy2002]

@bernd-ulmann Please decide, if you would like to do this in about the next 5-8 weeks (target: V1.6) or later (target: V1.7). Please use the label feature on the right side here in GitHub to label "V1.6" or "V1.7"

Branch: Please use develop

[sy2002]

@MJoergen Just in case you would like to apply your freshly honed QNICE assembly skills: Here is a relatively simple issue (simple as in "a few hours up to one day", or maybe a bit more) which would add quite some end-user convenience to the actual usage of QNICE-FPGA. The issue is assigned to Bernd, but he told me yesterday that he will not be able to do it in 1.6, so you could grab it, in case you want it: As usual, we are not in a hurry here. I was just stumbling over this one and saw, that this is a typical issue with the character of a good "first step" into the monitor architecture. So this is indeed just in case, you want to do some more assembly stuff now that you dived deeply into QNICE assembly while doing cpu_test.asm:-) If not: Don't worry, leave it with Bernd and V1.7 :-)

Right now he have the following inconvenience:

1. Boot the hardware on the Nexys with the STDIN switch to UART
2. Connect via serial and do somehing, e.g. press hfor help and read it on your serial device
3. Switch the STDIN switch to keyboard
4. You now cannot type yet on the keyboard, because UART$GETCHAR code link is still in a loop waiting for a character. Only after you send a key via UART/terminal program, then you can go on using the keyboard.
5. This is also true the other way round: if you started with STDIN at keyboard and then switched to UART, you are stuck in KBD$GETCHARcode link.