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 <sebastian.boe@nordicsemi.no>