fix syslinux splash regression [dep #186]
built upon MR #186 due to clash
fixes regression from 24658ce5 that resulted from overlooking something in reworking it from the original 2015 implementation.
with that change merging user files over default ones, if the user had provided a splash.png, the script would still convert the default splash.svg over the top of it.
this fixes the problem and restores the old logic, i.e. user provided splash.svg takes first priority, user provided splash.png second, default splash.svg third.