bootspec: store 'root' field in each bootspec entry we load

This 'root' field contains the root path of the partition we found the
snippet in. The 'kernel', 'initrd', 'efi', … fields are relative to this
path.

This becomes particularly useful later when we add support for loading
snippets from both the ESP and XBOOTLDR, but already simplifies the code
for us a bit in systemctl.

src/systemctl/systemctl.c

@@ -3519,7 +3519,7 @@ static int prepare_firmware_setup(void) {
 
 static int load_kexec_kernel(void) {
         _cleanup_(boot_config_free) BootConfig config = {};
-        _cleanup_free_ char *where = NULL, *kernel = NULL, *initrd = NULL, *options = NULL;
+        _cleanup_free_ char *kernel = NULL, *initrd = NULL, *options = NULL;
         const BootEntry *e;
         pid_t pid;
         int r;
@@ -3532,7 +3532,7 @@ static int load_kexec_kernel(void) {
         if (access(KEXEC, X_OK) < 0)
                 return log_error_errno(errno, KEXEC" is not available: %m");
 
-        r = find_default_boot_entry(arg_esp_path, &where, &config, &e);
+        r = find_default_boot_entry(arg_esp_path, &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)
@@ -3544,9 +3544,9 @@ static int load_kexec_kernel(void) {
                 return -EINVAL;
         }
 
-        kernel = path_join(where, e->kernel);
+        kernel = path_join(e->root, e->kernel);
         if (!strv_isempty(e->initrd))
-                initrd = path_join(where, *e->initrd);
+                initrd = path_join(e->root, *e->initrd);
         options = strv_join(e->options, " ");
         if (!options)
                 return log_oom();