drivers: eth: native_posix: Add support for setting ethernet MAC address

If CONFIG_ETH_NATIVE_POSIX_RANDOM_MAC=n and
CONFIG_ETH_NATIVE_POSIX_MAC_ADDR="", the MAC address can be set with a
net_mgmt call before the driver is initialized.

Signed-off-by: Björn Stenberg <bjorn@haxx.se>