Specify architecture in configuration

[AlexTalker]

Some distributions(like Arch Linux) have special variables for source files in recipes, so better solution could be specify target architecture for build call in this and similar services. Currently arch property is undef so build can't properly decide which sources you really need(which could be, for example, binary and have the same result name'). Please, fix it.

[AlexTalker]

As definition of Build::show says:

sub show {
  my ($conffile, $fn, $field, $arch) = @ARGV;
  my $cf = read_config($arch, $conffile);

You just can specify architecture as last argument.
I could do PR by myself but I need your help for properly integration with OBS service usage.