Fix panic if a board has a property that starts with menu

Author: silvanocerza

arduino/cores/packagemanager/loader.go

@@ -388,7 +388,7 @@ func (pm *PackageManager) loadBoards(platform *cores.PlatformRelease) error {
 	for boardID, boardProperties := range propertiesByBoard {
 		var board *cores.Board
 		for key := range boardProperties.AsMap() {
-			if !strings.HasPrefix(key, "menu") {
+			if !strings.HasPrefix(key, "menu.") {
 				continue
 			}
 			// Menu keys are formed like this:

test/testdata/platform_with_wrong_custom_board_options/boards.txt

@@ -60,3 +60,4 @@ altra.build.core=arduino
 altra.build.variant=standard
 altra.menu.cpu.atmega328=ATmega328P
 altra.menu.cpu.atmega168=ATmega168
+altra.menufoo=bar