nfs: skip bootpc when vfs.root.mountfrom is other than nfs

If "vfs.root.mountfrom" is set and the value is something other
than "nfs:*", it means the user doesn't want to mount root via nfs,
there's no reason to continue with bootpc

This fixes the powerpcspe kernel (MPC85XXSPE) that's compiled with
BOOTP_NFSROOT by default and gets stuck on bootpc/dhcp request loop
when no DHCP server is available on the network, even when user
specifies a local disk via "vfs.root.mountfrom" kernel parameter.

Reviewed by: imp
MFC after: 2 weeks
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D35098