nios2: zap nios2-generic board dir

As we use device tree to control u-boot now, the generic
board can be removed.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
diff --git a/configs/nios2-generic_defconfig b/configs/nios2-generic_defconfig
index 3d404b2..ea07b56 100644
--- a/configs/nios2-generic_defconfig
+++ b/configs/nios2-generic_defconfig
@@ -1,7 +1,7 @@
 CONFIG_NIOS2=y
+CONFIG_SYS_CONFIG_NAME="nios2-generic"
 CONFIG_DM_SERIAL=y
 CONFIG_DM_GPIO=y
-CONFIG_TARGET_NIOS2_GENERIC=y
 CONFIG_DEFAULT_DEVICE_TREE="3c120_devboard"
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_CPU=y