Re-Implement the DCD v2 processing.
Processing for the DCD write command went into a separate function enabling
the over-all DCD processing to handle check, nop and unlock commands as well.
The trivial NOP command is supported right away.
Further changes: put in some data consistancy checks and error handling, do
structured member access and proper endianess handling and direct error
messages to stderr.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>