build: target_config_check: make check more lenient

Always allow options set to builtin (=y) when modular setting (=m) is
expected. This can happen when a package is added explicitly (in a
target defintion or site.mk) that is also pulled in as a dependency of
another builtin package.

Fixes: 9e23534ec3 ("build: rework config generation")
Fixes: #2046

scripts/target_config_check.lua

@@ -9,18 +9,26 @@ local function fail(msg)
 	io.stderr:write(' * ', msg, '\n')
 end
 
-local function match_config(f)
-	for line in io.lines('openwrt/.config') do
-		if f(line) then
+local function match_config(expected, actual)
+	if expected == actual then
 		return true
 	end
+
+	if expected:gsub('=m$', '=y') == actual then
+		return true
 	end
 
 	return false
 end
 
 local function check_config(config)
-	return match_config(function(line) return line == config end)
+	for line in io.lines('openwrt/.config') do
+		if match_config(config, line) then
+			return true
+		end
+	end
+
+	return false
 end