luci-mod-admin-full: use sysupgrade -T to test images

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

modules/luci-mod-admin-full/luasrc/controller/admin/system.lua

@@ -178,13 +178,7 @@ function action_flashops()
 	local image_tmp   = "/tmp/firmware.img"
 
 	local function image_supported()
-		-- XXX: yay...
-		return ( 0 == os.execute(
-			". /lib/functions.sh; " ..
-			"include /lib/upgrade; " ..
-			"platform_check_image %q >/dev/null"
-				% image_tmp
-		) )
+		return (os.execute("sysupgrade -T %q >/dev/null" % image_tmp) == 0)
 	end
 
 	local function image_checksum()