Bootloader with SD card loading feature working

[cmarrin]

I've got an initial version of the bootloader for my Placid project running. This started as code from dwelch which does XModem uploads. The big feature I added was the ability to read a binary from an SD card with a FAT32 file system and run that. When the bootloader comes up it autoloads a file on the card named kernel.bin and runs it. If within 3 seconds you hit the space bar it will instead wait for an XModem upload.

I've only tested it on a RPi Zero W. I have a B+ on which I will try it soon. But it certainly won't work on, for instance a RPi 3. Still lots more work to do, but now I can start in earnest writing my kernel. Here it is:

https://github.com/cmarrin/placid

Comments and suggestions always welcome...