rockchip: mkimage: remove unused code-paths (spl_boot0 is now implied)

With all targets converted to generate prepadded images, this removes
the spl_boot0 field from our config structure and removes the unused
code-path (for images that are not prepadded): i.e. spl_boot0 is now
implied as 'true' and the code is specialised by removing the other
case.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>