Stop drivers checking DEVICE_EMAC

Checking DEVICE_EMAC is problematic - particularly for the Odin W2 where
apps have been shutting this off to disable the Wi-fi interface.

Make drivers check a locally-relevant flag instead, pending new
thoughts on how to achieve application/test-relevant flagging for
XXX:get_default_instance() being provided by a system.

However that is achieved, drivers do require a flag set purely by the
target - they mustn't be tripped up by an add-on module providing itself
as the system's default EMAC.

features/netsocket/emac-drivers/TARGET_Freescale/k64f_emac.cpp

@@ -42,7 +42,7 @@
 #include "netsocket/nsapi_types.h"
 #include "mbed_shared_queues.h"
 
-#if DEVICE_EMAC
+#ifdef ENET_BASE
 
 #include "fsl_phy.h"
 
@@ -614,7 +614,7 @@ MBED_WEAK EMAC &EMAC::get_default_instance() {
  * @}
  */
 
-#endif // DEVICE_EMAC
+#endif // ENET_BASE
 
 /* --------------------------------- End Of File ------------------------------ */
 

features/netsocket/emac-drivers/TARGET_STM/stm32xx_emac.cpp

@@ -7,7 +7,7 @@
 #include "mbed_shared_queues.h"
 #include "netsocket/nsapi_types.h"
 
-#if DEVICE_EMAC
+#ifdef ETH_BASE
 
 #include "stm32xx_emac_config.h"
 #include "stm32xx_emac.h"