nvram: add help message for nvram magic not found

The program would failed if nvram magic not found
in specific partition.

Signed-off-by: BangLang Huang <banglang.huang@foxmail.com>
(cherry picked from commit 69da83d9f1)

package/utils/nvram/src/cli.c

@@ -240,6 +240,7 @@ int main( int argc, const char *argv[] )
 			"	- Insufficient permissions to open mtd device\n"
 			"	- Insufficient memory to complete operation\n"
 			"	- Memory mapping failed or not supported\n"
+			"	- Nvram magic not found in specific nvram partition\n"
 		);
 
 		stat = 1;