loader warnings with LOADER_FDT_SUPPORT

Hi all,

The arm64 EFI loader is built with LOADER_FDT_SUPPORT, which is probably the
correct default, but when the loader has LOADER_FDT_SUPPORT set it emits some
unnecessarily scary warnings on many systems:

> No valid device tree blob found!
> WARNING! Trying to fire up the kernel, but no device tree blob found!

Since many systems legitimately don't have DTBs, can we remove this warning
and/or make it conditional on some check as to whether we think a DTB might
be needed?

In particular since this is the last thing the loader prints, it has sent a
few people down rabbit holes when the serial console in the loader works but
the serial console in the kernel doesn't, since they assume this message is
somehow related to the lack of further console output.

-- 
Colin Percival
FreeBSD Release Engineering Lead & EC2 platform maintainer
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid