dfu: Disable default calculation of CRC32

Patch (SHA1: bd694244)
dfu: Introduction of the "dfu_hash_algo" env variable for checksum method
setting

already introduced more generic handling of the crc32 calculation.
Up till now the CRC32 of received data was calculated unconditionally.
This patch changes this and from now - by default the crc32 is NOT
calculated anymore.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>