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.)

Author: rfinnie

scripts/build/binary_syslinux

@@ -313,8 +313,8 @@ then
 fi
 rm -f "${_TARGET}/splash.svg"
 
-# Tweak the grub theme to reuse the syslinux background image
-if [ -e binary/boot/grub/live-theme/theme.txt ]; then
+# Tweak the grub theme to reuse the syslinux background image if grub doesn't have its own
+if [ -e binary/boot/grub/live-theme/theme.txt ] && [ ! -e binary/boot/grub/splash.png ]; then
 	sed -i -e 's|^desktop-image:.*|desktop-image: "/isolinux/splash.png"|' \
 	-e 's|^title-text:.*|title-text: ""|' \
 	binary/boot/grub/live-theme/theme.txt