community/avr-libc: remove external dependencies, upgrade to 2.1.0

Previously, avr-libc downloaded an Ubuntu source package to extract header files from. Upon further inspection, it seems that upstream avr-libc started packaging most of those files, except for some redundant alias files with a single #include.

Files from the Ubuntu src were licensed under a different license than the rest of avr-libc, so IMO they should never have been packaged together. This merge removes this problem.