Blackfin: bf537-minotaur/bf537-srv1: undefine nfs when net is disabled

Fixes a build error due to new partial linking logic.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/include/configs/bf537-srv1.h b/include/configs/bf537-srv1.h
index 7e9dd36..e8024d7 100644
--- a/include/configs/bf537-srv1.h
+++ b/include/configs/bf537-srv1.h
@@ -156,6 +156,7 @@
 # define CONFIG_CMD_PING
 #else
 # undef CONFIG_CMD_NET
+# undef CONFIG_CMD_NFS
 #endif
 
 #define CONFIG_CMD_BOOTLDR