loader: lua: test that /boot exists first

Otherwise on fs like tftp where no directory listing is possible we fail
on the .dir method.

Reviewed by:	imp, kevans
MFC after:	2 weeks
Sponsored by:	Beckhoff Automation GmbH & Co. KG
Differential Revision:	https://reviews.freebsd.org/D33414

stand/lua/core.lua

@@ -229,6 +229,13 @@ function core.kernelList()
 		end
 	end
 
+	-- Do not attempt to autodetect if underlying filesystem
+	-- do not support directory listing (e.g. tftp, http)
+	if not lfs.attributes("/boot", "mode") then
+		autodetect = "no"
+		loader.setenv("kernels_autodetect", "NO")
+	end
+
 	-- Base whether we autodetect kernels or not on a loader.conf(5)
 	-- setting, kernels_autodetect. If it's set to 'yes', we'll add
 	-- any kernels we detect based on the criteria described.