Add a new kernel environment variable "boot.netif.mtu" which is used to set the MTU prior to mounting root via NFS.  This is required if the server supports a higher than default MTU because the client will not see the responses otherwise.
Add a new kernel environment variable "boot.netif.mtu" which is used to
set the MTU prior to mounting root via NFS.  This is required if the
server supports a higher than default MTU because the client will not
see the responses otherwise.

MFC after:	3 weeks

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