dfu: Introduction of the "dfu_checksum_method" env variable for checksum method setting

dfu: Introduction of the "dfu_checksum_method" env variable for checksum method setting

Up till now the CRC32 of received data was calculated unconditionally.
The standard crc32 implementation causes long delays when large images
were uploaded.

The "dfu_checksum_method" environment variable gives the opportunity to
enable on demand (when e.g. debugging) the crc32 calculation.
It can be done without need to recompile the u-boot binary.

By default the crc32 is not calculated.

Tests results:
400 MiB ums.img file
With 		crc32 calculation: 65 sec [avg 6.29 MB/s]
Without 		crc32 calculation: 25 sec [avg 16.17 MB/s]

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>