generator: run various generators only run on the host, not in initrd

These 5 generators only make sense on the host,not in the initrd, hence
if they end up in the initrd anyway, make them exit quickly.

src/system-update-generator/system-update-generator.c

@@ -5,6 +5,7 @@
 
 #include "fs-util.h"
 #include "generator.h"
+#include "initrd-util.h"
 #include "log.h"
 #include "path-util.h"
 #include "proc-cmdline.h"
@@ -61,6 +62,11 @@ static int run(const char *dest, const char *dest_early, const char *dest_late)
 
         assert_se(arg_dest = dest_early);
 
+        if (in_initrd()) {
+                log_debug("Skipping generator, running in the initrd.");
+                return EXIT_SUCCESS;
+        }
+
         r = generate_symlink();
         if (r <= 0)
                 return r;