serial uart now supports receive

I added receive support to my uart yesterday morning, but spent the afternoon with family so didn't get to do much with it. I was back to work today, but this evening I've knocked up an SREC loader:



This means that if I want to experiment with Z80, I can now upload new code in a matter of seconds instead of embedding it as a fake ROM inside the FPGA which requires about 7 minutes to compile and flash.

Obviously, it's still slow to work on the hardware side of things, but this will be a massive help testing out the flash chip, as well as getting large amounts of data onto it...