plymouth: introduce plymouth.enable=0 kernel command line switch

TODO

@@ -33,9 +33,7 @@ Features:
 
 * drop /.readahead on bigger upgrades with yum
 
-* add inode stat() check to readahead
-
-* plymouth.enable=0
+* add inode stat() check to readahead to suppress preloading changed files
 
 * include /usr/local/lib/tmpfiles.d in tmpfiles tool as search path
   (and prefix/lib, in case it isn't one of the listed ones)

units/plymouth-halt.service

@@ -10,6 +10,7 @@ Description=Show Plymouth Halt Screen
 After=getty@tty1.service prefdm.service plymouth-start.service
 Before=halt.service
 DefaultDependencies=no
+ConditionKernelCommandLine=!plymouth.enable=0
 
 [Service]
 ExecStart=/sbin/plymouthd --mode=shutdown

units/plymouth-kexec.service

@@ -10,6 +10,7 @@ Description=Show Plymouth Reboot with kexec Screen
 After=getty@tty1.service prefdm.service plymouth-start.service
 Before=kexec.service
 DefaultDependencies=no
+ConditionKernelCommandLine=!plymouth.enable=0
 
 [Service]
 ExecStart=/sbin/plymouthd --mode=shutdown

units/plymouth-poweroff.service

@@ -10,6 +10,7 @@ Description=Show Plymouth Power Off Screen
 After=getty@tty1.service prefdm.service plymouth-start.service
 Before=poweroff.service
 DefaultDependencies=no
+ConditionKernelCommandLine=!plymouth.enable=0
 
 [Service]
 ExecStart=/sbin/plymouthd --mode=shutdown

units/plymouth-reboot.service

@@ -10,6 +10,7 @@ Description=Show Plymouth Reboot Screen
 After=getty@tty1.service prefdm.service plymouth-start.service
 Before=reboot.service
 DefaultDependencies=no
+ConditionKernelCommandLine=!plymouth.enable=0
 
 [Service]
 ExecStart=/sbin/plymouthd --mode=shutdown

units/plymouth-start.service

@@ -14,6 +14,7 @@ Before=systemd-ask-password-plymouth.service
 
 # Dracut informs us with this flag file if plymouth is already running
 ConditionPathExists=!/run/plymouth/pid
+ConditionKernelCommandLine=!plymouth.enable=0
 
 [Service]
 ExecStart=/sbin/plymouthd --mode=boot --pid-file=/run/plymouth/pid