mkimage: Default to adding a crc32 hash with '-f auto'

This option currently does not add any sort of hash to the images in the
FIT.

Add a hash node requesting a crc32 checksum, which at least provides some
protection.

The crc32 value is easily ignored (e.g. in SPL) if not needed. and takes
up only about 48 bytes per image, including overhead.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Denk <wd@denx.de>