uboot-imx

net: sandbox: Add a priv ptr for tests to use

Tests need to be able to pass their "unit test state" to the handlers
where asserts are evaluated. Add a function that allows the tests to set
this private data on the sandbox eth device.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>