bootctl: split-out entry token related definitions into boot-entry.[ch]

No functional change, just preparation for later commits.
These can be used in kernel-install later.

Note, unlike the our usual coding style, the arguments for
boot_entry_token_ensure() and parse_boot_entry_token_type() are
referenced, updated, and may freed, hence, always pass initialized
values. That's why they are not named as 'ret_xyz'.

src/shared/boot-entry.h

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <stdbool.h>
+
+#include "sd-id128.h"
+
+typedef enum BootEntryTokenType {
+        BOOT_ENTRY_TOKEN_MACHINE_ID,
+        BOOT_ENTRY_TOKEN_OS_IMAGE_ID,
+        BOOT_ENTRY_TOKEN_OS_ID,
+        BOOT_ENTRY_TOKEN_LITERAL,
+        BOOT_ENTRY_TOKEN_AUTO,
+} BootEntryTokenType;
+
+bool boot_entry_token_valid(const char *p);
+
+int boot_entry_token_ensure(
+                const char *root,
+                const char *etc_kernel,   /* will be prefixed with root, typically /etc/kernel. */
+                sd_id128_t machine_id,
+                BootEntryTokenType *type, /* input and output */
+                char **token);            /* output, but do not pass uninitialized value. */
+
+int parse_boot_entry_token_type(const char *s, BootEntryTokenType *type, char **token);