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.)

You'll want to bump the "Timeout" from 1h to 2h under "General pipelines" on https://salsa.debian.org/rfinnie/live-build/-/settings/ci_cd

Once you've done that, click "Retry" on https://salsa.debian.org/rfinnie/live-build/-/jobs/1395579

Ah thanks, didn't know the timeout was configurable. Updated and run successfully.

merged