Move bootctl to its own source subdirectory

It's been split into a bunch of files and deserves its own subdirectory
similarly to systemctl.

src/bootctl/bootctl-random-seed.h

@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+int install_random_seed(const char *esp);
+
+int verb_random_seed(int argc, char *argv[], void *userdata);