kconfig: make default prompt of mainmenu less specific

If "mainmenu" is not specified, "Linux Kernel Configuration" is used
as a default prompt.

Given that Kconfig is used in other projects than Linux, let's use
a more generic prompt, "Main menu".
Suggested-by: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/tests/no_write_if_dep_unmet/expected_config

 #
 # Automatically generated file; DO NOT EDIT.
-# Linux Kernel Configuration
+# Main menu
 #
 # CONFIG_A is not set

scripts/kconfig/zconf.y

@@ -532,7 +532,7 @@ void conf_parse(const char *name)
 
 	if (!menu_has_prompt(&rootmenu)) {
 		current_entry = &rootmenu;
-		menu_add_prompt(P_MENU, "Linux Kernel Configuration", NULL);
+		menu_add_prompt(P_MENU, "Main menu", NULL);
 	}
 
 	menu_finalize(&rootmenu);