cmake: Cleanup ethernet include directory handling
cmake: Cleanup ethernet include directory handling

The zephyr/subsys/net/l2 include directory has been added through the
'zephyr_library_' API to modify the 'zephyr' library, when the
'zephyr_' API should have been used.

This patch fixes this problem. Using 'zephyr_library_' in this context
works by accident when 'zephyr' is the current library but has no
guarantees of working in the future.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

Vudentz/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr