generator: add root=off to explicitly turn search for rootfs off

This is useful when booting into storage target mode, where we do not
want to enter a root fs, we just want to stay forever in the initrd,
and hence there's value in not even generating and jobs to find the
rootfs

src/shared/generator.c

@@ -1114,6 +1114,10 @@ GptAutoRoot parse_gpt_auto_root(const char *value) {
                 return GPT_AUTO_ROOT_DISSECT_FORCE;
         }
 
-        log_debug("Disabling root partition auto-detection, root= is neither unset, nor set to 'gpt-auto', 'gpt-auto-force', 'dissect' or 'dissect-force'.");
+        if (streq(value, "off"))
+                log_debug("Disabling root partition auto-detection, root= handling is explicitly turned off.");
+        else
+                log_debug("Disabling root partition auto-detection, root= is neither unset, nor set to 'gpt-auto', 'gpt-auto-force', 'dissect' or 'dissect-force'.");
+
         return GPT_AUTO_ROOT_OFF;
 }