ostadmin: add update-kernel command to regenerate initramfs and boot config

This command regenerates kernel-dependent files. It is meant to
be run after kernel upgrades in the host distribution, to keep
the ostree installation running, and attempts to figure out the
right version automatically (and without changing the ostree checkout)

src/ostadmin/main.c

@@ -32,6 +32,7 @@
 static OtAdminBuiltin builtins[] = {
   { "deploy", ot_admin_builtin_deploy, 0 },
   { "init", ot_admin_builtin_init, 0 },
+  { "update-kernel", ot_admin_builtin_update_kernel, 0 },
   { NULL }
 };