kernel-install,bootctl: unify the config parsing procedure

Fixes https://github.com/systemd/systemd/issues/32992.

src/shared/kernel-config.h

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+int load_kernel_install_conf(
+                const char *root,
+                const char *conf_root,
+                char **ret_machine_id,
+                char **ret_boot_root,
+                char **ret_layout,
+                char **ret_initrd_generator,
+                char **ret_uki_generator);