Instead copy grub theme to /boot during postinst

The goal is to not break ARM images using vfat for /boot while
still providing the theme in /boot so that it can be available
even when /usr/share/ is not available.

Gbp-Dch: Full

share/grub/themes/kali/theme.txt

@@ -1,7 +1,7 @@
 
 # Global Property
 title-text: ""
-desktop-image: "/boot/grub/themes/kali/grub-16x9.png"
+desktop-image: "/usr/share/grub/themes/kali/grub-16x9.png"
 desktop-color: "#ff0000"
 terminal-font: "Unifont Regular 16"
 terminal-left: "0"