Fixing problems with generate_avr_firmware and generate_avr_library

I had a problem with generate_avr_firmware when I needed to use the HDRS and AFLAGS options.
The content of these options got mangled into the SRCS option. 

After looking through the code I found out that the variables containing the HDRS, LIBS, AFLAGS option lists are concatenated wrong. So I changed the method to use list functions provided by cmake.
This helped me and the variables seem to be correct now.