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

Fixes a build error due to new partial linking logic.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>

include/configs/bf537-minotaur.h

@@ -156,6 +156,7 @@
 # define CONFIG_CMD_PING
 #else
 # undef CONFIG_CMD_NET
+# undef CONFIG_CMD_NFS
 #endif
 
 #define CONFIG_CMD_BOOTLDR

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