faster submodule update

Submodules like `git` contain too much change history, which is not necessary to compile the program.  
Speeding up the setup significantly as well as saving space on the device is important.

README.md

@@ -92,7 +92,7 @@ How to Build
 **Initialize Submodules**
 
     git submodule init
-    git submodule update
+    git submodule update --depth 1
 
 **Build OpenSSL**