Update CDC Class Driver character stream functions to use the correct avr-libc return codes for errors and EOF.

Fix pointer arithmetic on void byte buffers by explicitly typecasting the buffer pointers to uint8_t* before altering them.

Add link to third-party signed version of the official FLIP DFU driver for 64-bit Windows OSes to the DFU bootloader readme file.

Add HTML anchor links to URLs in the documentation to ensure that they are rendered correctly in the generated documentation.