Fix info message for legacy command

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
Fixes #60
1 job for master in 25 seconds (queued for 2 seconds)