(This bug originates from https://chromium-review.googlesource.com/c/62895.)

When Depthcharge writes to flash through EC, it adds padding to make the data size equal to write_block_size of the spi flash. Thought this might be necessary to do for a certain board (Pit as described in the CL), this fails if we're writing to the end of the spi flash because EC checks whether (offset + size) exceeds the flash size or not (legitimately).

I think we should play this trick (padding to write block size) on each EC. That is, if EC receives a short packet, it should add padding by itself if it's required.