net: nfs: make NFS_TIMEOUT configurable

NFS_TIMEOUT is constant value defined in net/nfs.c. But sometimes it needs to adjust.
This patch enables to override NFS_TIMEOUT by defining CONFIG_NFS_TIMEOUT in a board specific config file.

Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
diff --git a/README b/README
index dac46f3..fb9d904 100644
--- a/README
+++ b/README
@@ -2149,6 +2149,13 @@
 
 		Timeout waiting for an ARP reply in milliseconds.
 
+		CONFIG_NFS_TIMEOUT
+
+		Timeout in milliseconds used in NFS protocol.
+		If you encounter "ERROR: Cannot umount" in nfs command,
+		try longer timeout such as
+		#define CONFIG_NFS_TIMEOUT 10000UL
+
 - Command Interpreter:
 		CONFIG_AUTO_COMPLETE