fix grub background/theme not shown on systems installed with full di…

…sk encryption

- update-grub includes /usr/share/desktop-base/...png in grub.cfg but the file cannot be acessed by grub when the root filesystem is not unlocked yet
- copy /usr/share/plymouth/themes/homeworld/plymouth_background_homeworld.png to /boot/grub/ so that update-grub prefers it over all other possible sources in /etc/grub.d/05_debian_theme
- fixes #94

LICENSE

@@ -1,6 +1,6 @@
 - `*`: GNU GPL-3.0
 - `config/includes.chroot/usr/share/backgrounds/futureprototype-darker-4096x2160`.svg: (c) 2018 Alex Makas, GNU GPL-3.0
-- `config/includes.chroot/usr/share/backgrounds/homeworld-darker-3840x2160.png, config/bootloaders/*/splash.png`: (c) 2020 Juliette Taka, CC-BY-SA-4.0 or GNU GPL-2.0
+- `config/includes.chroot/usr/share/backgrounds/homeworld-darker-3840x2160.png, config/bootloaders/*/splash.png, config/includes.chroot/boot/grub/splash.png`: (c) 2020 Juliette Taka, CC-BY-SA-4.0 or GNU GPL-2.0
 
 The ISO image resulting from the build includes software packages under
 various licenses. See `README/COPYING/AUTHORS/LICENSE/COPYRIGHT` files in