[menu] Add menu commands

Allow iPXE scripts to create menus.  For example:

    #!ipxe

    menu iSCSI boot demonstration
    item install         Install Fedora to ${root-path}
    item --default boot  Boot from ${root-path}
    item shell           Enter iPXE shell
    item exit            Exit to BIOS
    choose label && goto ${label}

    :boot
    sanboot ${root-path}

    :install
    sanhook ${root-path}
    chain http://${next-server}/fedora.ipxe

    :shell
    shell

    :exit

Inspired-by: Robin Smidsrød <robin@smidsrod.no>
Tested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/config.c

@@ -223,6 +223,9 @@ REQUIRE_OBJECT ( dhcp_cmd );
 #ifdef SANBOOT_CMD
 REQUIRE_OBJECT ( sanboot_cmd );
 #endif
+#ifdef MENU_CMD
+REQUIRE_OBJECT ( menu_cmd );
+#endif
 #ifdef LOGIN_CMD
 REQUIRE_OBJECT ( login_cmd );
 #endif