This allows us to invoke the build with a custom target (for the VMs
that use the {target} format string specifier). Currently OpenBSD is
still hardwired due to problems running check.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>