luci-app-attendedsysupgrade: fix missing efi path

If running on a non EFI system, the file `/sys/firmware/efi` is not
available and therefore results in an error of `fs.stat`. Wrap it with
`L.resolveDefault` to avoid the error message and make it work on
non-EFI devices again.

Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit 65266c490a)

applications/luci-app-attendedsysupgrade/htdocs/luci-static/resources/view/attendedsysupgrade/overview.js

@@ -401,7 +401,7 @@ return view.extend({
 		return Promise.all([
 			L.resolveDefault(callPackagelist(), {}),
 			L.resolveDefault(callSystemBoard(), {}),
-			fs.stat("/sys/firmware/efi"),
+			L.resolveDefault(fs.stat("/sys/firmware/efi"), null),
 			fs.read("/proc/mounts"),
 			uci.load('attendedsysupgrade'),
 		]);