OSBS will reject no scratch builds with arch_override.
When the option is not specified in Pungi, it would do `"".split(" ")`
to get list of arches, which returns a list with empty string instead of
an empty list.
With this fixed, it might be possible to have multiple images match the
spec (unless arch is used in the filter). To fix that, we can replace
arch with $basearch variable.
JIRA: RHELCMP-3824
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>
535034ef91