Clean up PowerPC loader(8) build config.

Turn off TFTP support by default: when both TFTP and NFS are enabled in the
loader, strange interactions occur in the pure netbooting scenario (i.e.
loader is TFTP-ed, kernel+world mounted over NFS), leading to very slow access
to the NFS-exported files.

Reviewed by: grehan
Approved by: cognet (mentor)