plymouth-ignore-serial-consoles: adjusting the method from marmarek suggestion

https://github.com/fepitre/qubes-linux-kernel/commit/af674124d6da121e285baf2b5b5ee412c8254c7e
(cherry picked from commit e69b02be)

kernel.spec.in

@@ -430,8 +430,10 @@ done
 
 # 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
+if [ -f /etc/default/grub ]; then
+    if ! grep -q plymouth.ignore-serial-consoles /etc/default/grub; then
+        echo 'GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX plymouth.ignore-serial-consoles"' >> /etc/default/grub
+    fi
 fi
 
 # grubby (used by new-kernel-pkg) do not understand xen entries in grub2 config