configure: modify default of building rombios

configure: modify default of building rombios

The tools/configure script will default to build rombios if qemu
traditional is enabled. If rombios is being built, ipxe will be built
per default, too.

This results in rombios and ipxe no longer being built by default when
disabling qemu traditional.

Fix that be rearranging the dependencies:

- build ipxe by default
- build rombios by default if either ipxe or qemu traditional are
  being built

This modification prepares not building qemu traditional by default
without affecting build of rombios and ipxe.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-acked-by: Ian Jackson <iwj@xenproject.org>