If the dhcp server provided an interface-mtu option, transcribe the value

to the boot.netif.mtu env var, which will be picked up by pre-existing code
in nfs_mountroot() and used to configure the interface accordingly.

This should bring the same functionality when the bootp/dhcp work is done
by loader(8) as r297150 does for the in-kernel BOOTP case.

sys/boot/common/dev_net.c

@@ -169,6 +169,12 @@ net_open(struct open_file *f, ...)
 		setenv("boot.netif.gateway", inet_ntoa(gateip), 1);
 		setenv("boot.nfsroot.server", inet_ntoa(rootip), 1);
 		setenv("boot.nfsroot.path", rootpath, 1);
+		if (intf_mtu != 0) {
+			char mtu[16];
+			sprintf(mtu, "%u", intf_mtu);
+			setenv("boot.netif.mtu", mtu, 1);
+		}
+
 	}
 	netdev_opens++;
 	f->f_devdata = &netdev_sock;

sys/boot/i386/libi386/pxe.c

@@ -310,6 +310,11 @@ pxe_open(struct open_file *f, ...)
 		    sprintf(temp, "%6D", bootplayer.CAddr, ":");
 		    setenv("boot.netif.hwaddr", temp, 1);
 		}
+		if (intf_mtu != 0) {
+			char mtu[16];
+			sprintf(mtu, "%u", intf_mtu);
+			setenv("boot.netif.mtu", mtu, 1);
+		}
 #ifdef LOADER_NFS_SUPPORT
 		printf("pxe_open: server addr: %s\n", inet_ntoa(rootip));
 		printf("pxe_open: server path: %s\n", rootpath);