Remove unnecessary files.

We only need some of the source code to build our own software.
This commit removes all the extra files to make it faster and easier use tarball
checkouts/downloads for our builds.
Increase H/W serial buffer sizes by one to have 64 bytes of useable space.
H/W serial - Comment out write overloads.

These integer overloads only ever write a single byte instead of the actual given value.
H/W Serial - Add clear() function to reset the rx buffers.
H/W Serial - Add peek(int) function to peek further into the rx buffer.
H/W Serial - Add checksum(int) function

Calculates checksum for the next n bytes in the rx buffer without modifying the buffer.
Fix some compilation warnings.
Fix inline assembly compilation error.

Wrong constraint was used for sbiw instruction.