stand/fdt: Avoid bailout when dtbo has no fixups

[kevans]

Summary

In the case of a simple dtbo where fragment uses target-path and the overlay contains no references, /__fixups__ will not be included by either our dtc or dtc from ports, but the file still has valid fragments to be applied. My single test fragment:

target-path = "/soc/ethernet";
__overlay__ {
    phy-mode = "rgmii-txid";
};

[kevans]

Further testing showed that one of the dt compilers I tried emitted empty /__symbols__ while the other just omitted /__symbols__ since it was empty. I don't know if omitting it is OK according to whatever spec we may follow, but allowing it doesn't seem harmful.