Don't try to copy or create symlinks in /boot

Fixes https://bugs.kali.org/view.php?id=6583

debian/kali-themes.postinst

@@ -87,10 +87,10 @@ if [ "$1" = "configure" ]; then
     # necessary account for grub being the active bootloader).
     if dpkg-query --list grub-efi* | grep -v "^... grub-efi[^[:space:]]*-bin" | grep -q "^[ih][HUFWti] " ; then
         echo "grub-efi* packages found, using 16/9 as default grub background ratio"
-        ln -sf /boot/grub/themes/kali/grub-16x9.png /boot/grub/themes/kali/background.png
+        cp /boot/grub/themes/kali/grub-16x9.png /boot/grub/themes/kali/background.png
     else
         echo "No grub-efi* package found, using 4/3 as default grub background ratio"
-        ln -sf /boot/grub/themes/kali/grub-4x3.png /boot/grub/themes/kali/background.png
+        cp /boot/grub/themes/kali/grub-4x3.png /boot/grub/themes/kali/background.png
     fi
     # Rebuild the grub configuration with our config changes
     if which update-grub >/dev/null; then

share/grub/themes/kali/background.png

@@ -1 +0,0 @@
-grub-16x9.png