installer: enable plymouth splash using late_command in preseed file

- adding a custom /etc/default/grub in the chroot breaks unattended installation (conffile conflict resolution)
- add the "splash" boot option as late as possible during installation, update grbu configuration in target
- https://unix.stackexchange.com/questions/438901/reconfigure-grub-in-preseed-late-command
- fixes #87

config/includes.installer/preseed.cfg

@@ -481,6 +481,9 @@ d-i finish-install/reboot_in_progress note
 # directly, or use the apt-install and in-target commands to easily install
 # packages and run commands in the target system.
 #d-i preseed/late_command string apt-install zsh; in-target chsh -s /bin/zsh
+d-i preseed/late_command string \
+    in-target sed -i 's/GRUB_CMDLINE_LINUX_DEFAULT="quiet"/GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"/'; \
+    in-target grub-mkconfig -o /boot/grub/grub.cfg
 
 # By default, the system's locate database will be updated after the
 # installer has finished installing most packages. This may take a while, so