Bootloader protocol optimization: Allow "excess" data transmission in UPD_SEND_DATA messages

[dallmair]

At the moment, the data transmission in UPD_SEND_DATA messages must be page-aligned, i.e. an UPD_SEND_DATA message must only contain data of a single page. The message must not contain more data than can fit into the current page.

Proposal is to lift this restriction and allow transmission of the first bytes of the following page together with the last bytes of the previous page. For this, the buffer size must be increased from RAM_BUFFER_SIZE to RAM_BUFFER_SIZE + MAX_BYTES_PER_SEND_DATA_MESSAGE.

With Standard Frames, we currently transmit 21 messages with 12 payload bytes each and another message with the final 4 bytes of the page. Probably, the savings are therefore only measurable when looking at the complete flash sequence.
However, things get way more interesting with Extended Frames. There, we can transmit up to 252 bytes in a frame and then need yet another (Standard) frame with all the LL_ACK/T_ACK/LL_ACK ping-pong just for the final 4 bytes. And that for every page... :(