[DELL]: FTOS to SONiC fast conversion fixes (#4807)

While migrating to SONiC 20181130, identified a couple of issues:
1. union-mount needs /host/machine.conf parameters for vendor specific checks : however, in case of migration, the /host/machine.conf is extracted from ONIE only in https://github.com/Azure/sonic-buildimage/blob/master/files/image_config/platform/rc.local#L127.
2. Since grub.cfg is updated to have net.ifnames=0 biosdevname=0, 70-persistent-net.rules changes are no longer required.

files/image_config/platform/rc.local

@@ -82,10 +82,6 @@ update_mgmt_interface_macaddr() {
 
     log_migration "eth0 mac in EEPROM after update:"
     ethtool -e eth0 offset $ethtool_offset length 6 >> /host/migration/migration.log
-
-    # Update the 70-persistent-net.rules with the new mac for eth0
-    log_migration "/etc/udev/rules.d/70-persistent-net.rules : replacing $old_mac with $new_mac for eth0"
-    sed -i "/eth0/ s/ATTR{address}==\"$old_mac\"/ATTR{address}==\"$new_mac\"/g" /etc/udev/rules.d/70-persistent-net.rules
 }
 
 firsttime_exit() {