VxWorks: fixup MAC address for VxWorks

VxWorks 7 kernels retrieve 'local-mac-addr' from dtb and use
that for NIC MAC address. As a result, when booting the same
kernel image on multiple boards, there will be address
conflicts.

So fixup MAC address when booting VxWorks 7 kernels

Signed-off-by: Miao Yan <yanmiaobest@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>