Add Plymouth boot splash plugin

Manage the plymouthd lifecycle across boot, switch_root, and shutdown.
Activated by the "splash" kernel command line argument.

plugins/Makefile.am

@@ -29,6 +29,10 @@ if BUILD_NETLINK_PLUGIN
 libplug_la_SOURCES += netlink.c
 endif
 
+if BUILD_PLYMOUTH_PLUGIN
+libplug_la_SOURCES += plymouth.c
+endif
+
 if BUILD_RESOLVCONF_PLUGIN
 libplug_la_SOURCES += resolvconf.c
 endif
@@ -76,6 +80,10 @@ if BUILD_NETLINK_PLUGIN
 pkglib_LTLIBRARIES += netlink.la
 endif
 
+if BUILD_PLYMOUTH_PLUGIN
+pkglib_LTLIBRARIES += plymouth.la
+endif
+
 if BUILD_RESOLVCONF_PLUGIN
 pkglib_LTLIBRARIES += resolvconf.la
 endif