fdt_opt == 'disabled' is going to give an error if libfdt is required
by any target, so catch that immediately.  For fdt_opt == 'enabled',
instead, do not check immediately whether the internal libfdt is present.
Instead do the check after ascertaining that libfdt is absent or too old.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>