The CMD_DCD_SKIP case misses a break statement.

Add it.

Fixes: 254c0080 ("tools: imx8image: add possibility to skip dcd")
Addresses-Coverity-ID: 514648: Control flow issues (MISSING_BREAK)
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>