fixed issues in Config.in and at91ariag25.c

I think you may have accidentally deleted a few lines out of Config.in while merging a few hours ago.

The project wouldn't compile without the changes I just made to at91ariag25.c. The dbgu.h file changed to usart.h. I also opportunistically changed a few macros for ease of use. I verified that the project would compile and boot - I'm using the terminal on my ariag25 right now using the boot.bin file created with the changes in these commits.