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The current code checks that the transferred size is matching the size
reported in the image header.

Unfortunately, the transferred image might be padded, which doesn't change
anything at the functional level, but will make that check trigger since
the actual image will be smaller than the transferred data.

Change that logic to first check that the transferred size isn't less that
the header image size, which will still be an error, and then check for the
CRC of the image itself. This will prove to be an more robust integrity
check than what we have right now anyway.

The CRC used in the image header is the CRC32 algorithm, that is
implemented in the zlib, which is installed on most devices on the planet,
so we can just use that implementation instead of rolling our own.
Tested-by: Frank Kunz <mailinglists@kunz-im-inter.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>