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>