binary_syslinux: Do not modify grub theme.txt if grub/splash.png exists
If grub/splash.png exists, assume the configuration editor intends to have a grub-specific splash.png, and do not modify theme.txt. But if syslinux has the only known splash.png, use it for both syslinux and grub.
(This allows for a hybrid image where the grub side can have e.g. a 16:9 1920x1080 splash.png which gets grub is capable of automatically scaling, while the syslinux side has a 640x480 splash.png which effectively must be this fixed size.)