Move logic to find default sd-boot entry from systemctl to shared

In preparation for use in other places. No functional change.

src/shared/bootspec.c

@@ -598,3 +598,36 @@ found:
 
         return 0;
 }
+
+int find_default_boot_entry(
+                const char *esp_path,
+                char **esp_where,
+                BootConfig *config,
+                const BootEntry **e) {
+
+        _cleanup_free_ char *where = NULL;
+        int r;
+
+        assert(config);
+        assert(e);
+
+        r = find_esp_and_warn(esp_path, false, &where, NULL, NULL, NULL, NULL);
+        if (r < 0)
+                return r;
+
+        r = boot_entries_load_config(where, config);
+        if (r < 0)
+                return log_error_errno(r, "Failed to load bootspec config from \"%s/loader\": %m", where);
+
+        if (config->default_entry < 0) {
+                log_error("No entry suitable as default, refusing to guess.");
+                return -ENOENT;
+        }
+
+        *e = &config->entries[config->default_entry];
+
+        if (esp_where)
+                *esp_where = TAKE_PTR(where);
+
+        return 0;
+}

src/shared/bootspec.h

@@ -51,3 +51,5 @@ static inline const char* boot_entry_title(const BootEntry *entry) {
 }
 
 int find_esp_and_warn(const char *path, bool unprivileged_mode, char **ret_path, uint32_t *ret_part, uint64_t *ret_pstart, uint64_t *ret_psize, sd_id128_t *ret_uuid);
+
+int find_default_boot_entry(const char *esp_path, char **esp_where, BootConfig *config, const BootEntry **e);

src/systemctl/systemctl.c

@@ -3455,21 +3455,12 @@ static int load_kexec_kernel(void) {
         if (access(KEXEC, X_OK) < 0)
                 return log_error_errno(errno, KEXEC" is not available: %m");
 
-        r = find_esp_and_warn(arg_esp_path, false, &where, NULL, NULL, NULL, NULL);
-        if (r == -ENOKEY) /* find_esp_and_warn() doesn't warn about this case */
+        r = find_default_boot_entry(arg_esp_path, &where, &config, &e);
+        if (r == -ENOKEY) /* find_default_boot_entry() doesn't warn about this case */
                 return log_error_errno(r, "Cannot find the ESP partition mount point.");
-        if (r < 0) /* But it logs about all these cases, hence don't log here again */
-                return r;
-
-        r = boot_entries_load_config(where, &config);
         if (r < 0)
-                return log_error_errno(r, "Failed to load bootspec config from \"%s/loader\": %m", where);
-
-        if (config.default_entry < 0) {
-                log_error("No entry suitable as default, refusing to guess.");
-                return -ENOENT;
-        }
-        e = &config.entries[config.default_entry];
+                /* But it logs about all these cases, hence don't log here again */
+                return r;
 
         if (strv_length(e->initrd) > 1) {
                 log_error("Boot entry specifies multiple initrds, which is not supported currently.");