bootspec: generalize "addon" logic for all kinds of extra sidecars

Let's pick up all kinds of sidecars and show them, not just addons

This also fixes some issues regarding "root" directory handling.
In one context we'd resolve a directory claiming it was a "root", but it
wasn't.

Implements: https://github.com/uapi-group/specifications/pull/212

test/units/TEST-87-AUX-UTILS-VM.bootctl.sh

@@ -374,7 +374,7 @@ testcase_00_secureboot() {
     bootctl status | grep "Secure Boot: enabled" >/dev/null
 
     # Ensure the addon is fully loaded and parsed
-    bootctl status | grep "global-addon: loader/addons/test.addon.efi" >/dev/null
+    bootctl status | grep "extra: /boot//loader/addons/test.addon.efi" >/dev/null
     bootctl status | grep "cmdline" | grep addonfoobar >/dev/null
     grep -q addonfoobar /proc/cmdline
 }