samples: cdc_acm: add config overlay for cdc_acm+dfu

Add config overlay for cdc_acm+dfu composite configuration.
Manual configuration over menuconfig may be error-prone for
the user because then FLASH_LOAD_OFFSET and FLASH_LOAD_SIZE
should also to be set to appropriate values.

Signed-off-by: Johann Fischer <j.fischer@phytec.de>