tools: mkimage: fix sfspl_image_extract_subimage()

Do not leak file descriptor if writing fails.
Correct the error text if opening a file fails.

Addresses-Coverity-ID: 467054 Resource leaks
Fixes: 64fd30d367a1 ("tools: mkimage: Add StarFive SPL image support")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

tools: mkimage: Add StarFive SPL image support

The StarFive JH7110 base boards require a header to be prefixed to the SPL
binary image. This has previously done with a vendor tool 'spl_tool'
published under a GPL-2-or-later license. Integrate this capability into
mkimage.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: Chanho Park <chanho61.park@samsung.com>
Tested-by: Milan P. Stanić <mps@arvanta.net>

tools: mkimage: Add StarFive SPL image support

The StarFive JH7110 base boards require a header to be prefixed to the SPL
binary image. This has previously done with a vendor tool 'spl_tool'
published under a GPL-2-or-later license. Integrate this capability into
mkimage.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: Chanho Park <chanho61.park@samsung.com>
Tested-by: Milan P. Stanić <mps@arvanta.net>