dumpimage: fix handling of StarFive SPL too long

dumpimage: fix handling of StarFive SPL too long

The header of the StarFive U-Boot SPL file u-boot-spl.normal.out has a
field indicating the payload size. When copying U-Boot SPL from a
partition the copied file might be too long.

Currently in this situation a misleading error message 'Incorrect CRC32' is
written.

We must use the payload size and not the file size when calculating the
CRC32.

Write a warning if the file is too long indicating the correct size. This
enables the user to truncate the file accordingly.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>