blackfin: fix build error by adding CONFIG_BFIN_SERIAL

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/include/configs/blackvme.h b/include/configs/blackvme.h
index cd37f9a..6e5774c 100644
--- a/include/configs/blackvme.h
+++ b/include/configs/blackvme.h
@@ -177,6 +177,7 @@
 #define CONFIG_BAUDRATE		57600
 #define CONFIG_LOADS_ECHO	1
 #define CONFIG_UART_CONSOLE	0
+#define CONFIG_BFIN_SERIAL
 
 /*
  * U-Boot environment variables. Use "printenv" to examine.