(breaking): use crc16 for writeblock command instead of cks_add8

[fenugrec]

cks_add8() is dumb and only used for <SID_FLASH> <SIDFL_WB> ... <CRC> frames. Using the crc16 code we already have anyway would eliminate some code, at the cost of 1 more byte to transfer per chunk. This change would need to be done in nisprog at the same time. ( https://github.com/fenugrec/nisprog/issues/12)