loader.efi: Fix LOADER_NET_SUPPORT=no builds

We include too many references to networking things in the
non-networking build for loader.efi. Ifdef out netserver command when we
are disabling network support to resolve.

PR: 277528
MFC After: 2 days
Reviewed by: imp

(cherry picked from commit 47ef2a131091508e049ab10cad7f91a3c1342cd9)