eNET: Fix undefined reference to `monitor_flash_len'

commit cfbe861506 removed the definition of
monitor_flash_len from the eNET which was not picked up due to extensive
use of the SRAM configuration target for testing

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>

board/eNET/eNET.c

@@ -35,6 +35,8 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
+unsigned long monitor_flash_len = CONFIG_SYS_MONITOR_LEN;
+
 static void enet_timer_isr(void);
 static void enet_toggle_run_led(void);
 static void enet_setup_pars(void);