Fix info message for legacy command

archiso/mkarchiso: Change the way show_config() displays information about the build environment, as displaying a profile directory as a legacy command to mkarchiso is confusing. The function now prints a deprecation message if '$command_name' is not a directory (i.e. not a profile). Remove 'command' prefix from help output for legacy commands (the prefix is only in use for internal functions).

Relates to !69 (merged) Fixes #60 (closed)