kernel-config: entry_name_format in install.conf

Extend load_kernel_install_conf_at() to parse the new entry_name_format=
setting from install.conf. This plumbing only, no behavioral change yet.
Existing callers pass NULL for the new parameter.

The entry_name_format= setting will allow configuring the boot entry
filename format using specifiers, replacing the current hardcoded
$ENTRY_TOKEN-$KERNEL_VERSION pattern. This is useful for image-based OS
builds (e.g. via mkosi, or image-builder) that require versioned UKI
filenames in the ESP for sysupdate A/B updates.

Having this configuration option allows to do that directly at build
time without having to side-step kernel-install and moving files around
manually.

src/kernel-install/kernel-install.c

@@ -467,7 +467,8 @@ static int context_load_install_conf(Context *c) {
                         &boot_root,
                         &layout,
                         &initrd_generator,
-                        &uki_generator);
+                        &uki_generator,
+                        /* ret_entry_name_format= */ NULL);
         if (r <= 0)
                 return r;