dfu: Extract common DFU code to handle "dfu_alt_info" environment variable

New dfu_init_env_entities() function has been extracted from cmd_dfu.c and
stored at dfu core.

This is a dfu centric code, so it shall be processed in the core.

Change-Id: I756c5de922fa31399d8804eaadc004ee98844ec2
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Heiko Schocher <hs@denx.de>