gluon-core: gluon.platform: allow omitting target and subtarget

Allow wildcard matches on all subtargets of a target, or a full wildcard
always returning true when no arguments are passed.

package/gluon-core/luasrc/usr/lib/lua/gluon/platform.lua

@@ -7,11 +7,11 @@ local M = setmetatable({}, {
 })
 
 function M.match(target, subtarget, boards)
-	if M.get_target() ~= target then
+	if target and M.get_target() ~= target then
 		return false
 	end
 
-	if M.get_subtarget() ~= subtarget then
+	if subtarget and M.get_subtarget() ~= subtarget then
 		return false
 	end