drivers: wifi: Prevent CMake warnings for out-of-tree drivers

When Ethernet or Wi-Fi subsystems are enabled, but no driver source
files are provided in the Zephyr source tree (which is the case for
out-of-tree drivers), CMake throws warnings about empty libs, for
example:

  No SOURCES given to Zephyr library: drivers__ethernet
  No SOURCES given to Zephyr library: drivers__wifi

This commit sets ALLOW_EMPTY property for those libraries, to allow for
seamless out-of-tree drivers integration.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

drivers/ethernet/CMakeLists.txt

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 zephyr_library()
+zephyr_library_property(ALLOW_EMPTY TRUE)
 
 zephyr_library_sources_ifdef(CONFIG_ETH_GECKO
 	eth_gecko.c

drivers/wifi/CMakeLists.txt

@@ -1,6 +1,8 @@
 # SPDX-License-Identifier: Apache-2.0
 
 zephyr_library()
+zephyr_library_property(ALLOW_EMPTY TRUE)
+
 add_subdirectory_ifdef(CONFIG_WIFI_ESP_AT       esp_at)
 add_subdirectory_ifdef(CONFIG_WIFI_ESP32        esp32)
 add_subdirectory_ifdef(CONFIG_WIFI_ESWIFI       eswifi)