configs: disable emulators that require it if libfdt is not found

configs: disable emulators that require it if libfdt is not found

Since boards can express their dependency on libfdt and
system/device_tree.c, only leave TARGET_NEED_FDT if the target has a
hard dependency.

Those emulators will be skipped if libfdt is disabled, or if it
is "auto" and not found and --disable-download is passed; unless
the target is mentioned explicitly in --target-list, in which case
the build will fail.

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