firstboot: add kernel cmdline option to disable firstboot wizard

If booting with systemd.firstboot=0 the wizard will be skipped.

src/firstboot/firstboot.c

@@ -33,6 +33,7 @@
 #include "mkdir.h"
 #include "parse-util.h"
 #include "path-util.h"
+#include "proc-cmdline.h"
 #include "random-util.h"
 #include "string-util.h"
 #include "strv.h"
@@ -825,6 +826,7 @@ static int parse_argv(int argc, char *argv[]) {
 }
 
 int main(int argc, char *argv[]) {
+        _cleanup_free_ char *enabled = NULL;
         int r;
 
         r = parse_argv(argc, argv);
@@ -837,6 +839,17 @@ int main(int argc, char *argv[]) {
 
         umask(0022);
 
+        r = get_proc_cmdline_key("systemd.firstboot=", &enabled);
+        if (r < 0)
+                return r;
+        if (r > 0) {
+                r = parse_boolean(enabled);
+                if (r == 0)
+                        goto finish;
+                if (r < 0)
+                        log_warning_errno(r, "Failed to parse systemd.firstboot= kernel command line argument, ignoring: %s", enabled);
+        }
+
         r = process_locale();
         if (r < 0)
                 goto finish;