Add the function declarations of WEAK attribute to use LWIP on GR-LYCHEE

I added the function declarations of Ethernet functions that have a WEAK attribute. Although several Ethernet functions was called in rza1_emac.c, GR-LYCHEE don't have Ethernert feature. But there may be case that GR-LYCHEE uses LWIP feature.
In this case, since GR-LYCHEE will occur the build error, I addressed the error by defining the functions with a WEAK attribute. For reason of WEAK attribute, there is no influence in GR-PEACH and VK_RZ_A1H that have Ethernet feature.

features/FEATURE_LWIP/lwip-interface/lwip-eth/arch/TARGET_RZ_A1XX/rza1_emac.c

@@ -5,11 +5,73 @@
 #include "mbed_interface.h"
 #include "ethernet_api.h"
 #include "ethernetext_api.h"
+#include "platform/mbed_toolchain.h"
 
 #define RECV_TASK_PRI           (osPriorityNormal)
 #define PHY_TASK_PRI            (osPriorityNormal)
 #define PHY_TASK_WAIT           (200)
 
+WEAK int ethernetext_init(ethernet_cfg_t *p_ethcfg)
+{
+    return -1;
+}
+
+WEAK void ethernetext_start_stop(int32_t mode)
+{
+}
+
+WEAK int ethernetext_chk_link_mode(void)
+{
+    return NEGO_FAIL;
+}
+
+WEAK void ethernetext_set_link_mode(int32_t link)
+{
+}
+
+WEAK int ethernet_init(void)
+{
+    return -1;
+}
+
+WEAK void ethernet_free(void)
+{
+}
+
+WEAK int ethernet_write(const char *data, int size)
+{
+    return 0;
+}
+
+WEAK int ethernet_send(void)
+{
+    return 0;
+}
+
+WEAK int ethernet_receive(void)
+{
+    return 0;
+}
+
+WEAK int ethernet_read(char *data, int size)
+{
+    return 0;
+}
+
+WEAK void ethernet_address(char *mac)
+{
+}
+
+WEAK int ethernet_link(void)
+{
+    return 0;
+}
+
+WEAK void ethernet_set_link(int speed, int duplex)
+{
+}
+
+
 /* memory */
 static sys_sem_t recv_ready_sem;    /* receive ready semaphore */