hw/xtensa: require libfdt

All other boards require libfdt if it can be used (including for example
i386/x86_64), so change the "imply" to "select" and always allow -dtb
in qemu-system-xtensa.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2 files changed