Support WiFi as default network interface

This requires support for weak symbol override. Object files arcived in
static library don't always participate in linking, dependent on linker
and link order. The steps below pull in the override object file anyway
even though it comes from static library:

1. Add e.g. GCC linker option "--undefined=<LINK_FOO>"
2. Add <LINK_FOO> function with 'extern "C"' in override source file

See: https://stackoverflow.com/questions/42588983/what-does-the-gnu-ld-undefined-option-do

connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/CMakeLists.txt

@@ -12,3 +12,13 @@ target_include_directories(mbed-wifi
         .
         ./ESP8266
 )
+
+# Link override object file from static library
+if("MBED_CONF_ESP8266_PROVIDE_DEFAULT=1" IN_LIST MBED_CONFIG_DEFINITIONS)
+    if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
+        target_link_options(mbed-wifi
+            INTERFACE
+                LINKER:--undefined=LINK_ESP8266INTERFACE_CPP
+        )
+    endif()
+endif()

connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/ESP8266Interface.cpp

@@ -1149,6 +1149,20 @@ WiFiInterface *WiFiInterface::get_default_instance()
     return &esp;
 }
 
+/*
+ * With e.g. GCC linker option "--undefined=<LINK_FOO>", pull in this
+ * object file anyway for being able to override weak symbol successfully
+ * even though from static library. See:
+ * https://stackoverflow.com/questions/42588983/what-does-the-gnu-ld-undefined-option-do
+ *
+ * NOTE: For C++ name mangling, 'extern "C"' is necessary to match the
+ *       <LINK_FOO> symbol correctly.
+ */
+extern "C"
+void LINK_ESP8266INTERFACE_CPP(void)
+{
+}
+
 #endif
 
 void ESP8266Interface::refresh_conn_state_cb()

connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/CMakeLists.txt

@@ -21,3 +21,13 @@ target_sources(mbed-wifi
         mx_wifi/core/mx_wifi_ipc.c
         mx_wifi/core/mx_wifi_slip.c
 )
+
+# Link override object file from static library
+if("MBED_CONF_EMW3080B_PROVIDE_DEFAULT=1" IN_LIST MBED_CONFIG_DEFINITIONS)
+    if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
+        target_link_options(mbed-wifi
+            INTERFACE
+                LINKER:--undefined=LINK_EMW3080BINTERFACE_CPP
+        )
+    endif()
+endif()

connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/EMW3080BInterface.cpp

@@ -352,6 +352,20 @@ WiFiInterface *WiFiInterface::get_default_instance()
     static EMW3080BInterface emw;
     return &emw;
 }
+
+/*
+ * With e.g. GCC linker option "--undefined=<LINK_FOO>", pull in this
+ * object file anyway for being able to override weak symbol successfully
+ * even though from static library. See:
+ * https://stackoverflow.com/questions/42588983/what-does-the-gnu-ld-undefined-option-do
+ *
+ * NOTE: For C++ name mangling, 'extern "C"' is necessary to match the
+ *       <LINK_FOO> symbol correctly.
+ */
+extern "C"
+void LINK_EMW3080BINTERFACE_CPP(void)
+{
+}
 #endif /* MBED_CONF_EMW3080B_PROVIDE_DEFAULT */