plymouth: ignore serial console hvc0

It fixes splash screen issue QubesOS/qubes-issues#3849

kernel.spec.in

@@ -402,6 +402,12 @@ done
 %posttrans
 /bin/kernel-install add %{kernelrelease} /boot/vmlinuz-%{kernelrelease} || exit $?
 
+# with kernel-4.14+ plymouth detects hvc0 serial console and forces text boot
+# we simply make plymouth ignore it to recover the splash screen
+if ! grep -q '^GRUB_CMDLINE_LINUX.*plymouth.ignore-serial-consoles' /etc/default/grub; then
+    sed -i 's/^GRUB_CMDLINE_LINUX="[^"]*/& plymouth.ignore-serial-consoles/' /etc/default/grub
+fi
+
 # grubby (used by new-kernel-pkg) do not understand xen entries in grub2 config
 if [ -x /sbin/new-kernel-pkg -a -e /boot/grub2/grub.cfg ]; then
         grub2-mkconfig > /boot/grub2/grub.cfg