Platform configs silently override user configs

The change was introduced by @nirbheek in 98dae501

1. the context: _load_platform_config() is called after _load_cmd_config(filename) in Config.load(filename, vars)
2. logically, it should be in reverted order (the platform is the base of the user config), but the information about the platform, distro, arch, etc. is defined by the user config; this could be resolved by parsing the user config twice: first to guess the info for _load_platform_config(), and secondly to do a final job
3. the variants variable became Variants instance; before it was a list, it was possible to check with the help of 'name' in variants if the variant is set already by the user config
4. Since the change, Variants.override() is used in platform config files to unconditionally set variants; this silently overrides values possibly set by the user config

Back link: 98dae501 (comment 2478969)