linux-flavour.rst: Add Usage section

The linux flavour page contains information about the login and qemu
usage which is useful for all build targets. To avoid duplication and
more easily maintain changes to this information, separate this into
a section that can be referenced at need.

Signed-off-by: Luca Seritan <luca.seritan@huawei.com>

docs/build-flavours/linux-flavour.rst

@@ -42,11 +42,16 @@ You can test the image built for the qemux86-64 target by issuing:
 
     $ runqemu qemux86-64 oniro-image-base wic ovmf slirp
 
+.. _linux-flavour-usage:
+
+Usage
+*****
+
 After successful bootup, you will be dropped into a login shell:
 
 .. code-block:: console
 
-    qemux86-64 login:
+    oniro-linux-qemux86-64 login:
     
 The default login name is **oniro** with **oniro** as a password.
 
@@ -54,12 +59,13 @@ After login you will see the shell prompt:
 
 .. code-block:: console
 
-    root@qemux86-64:~#
+    oniro@oniro-linux-qemux86-64:~$
 
 To exit qemu, you can either shut down the system:
 
 .. code-block:: console
 
-    root@qemux86:~# poweroff -f
+    oniro@oniro-linux-qemux86-64:~$ sudo poweroff    
+    Password: oniro 
 
 or close qemu using a key combination: *Ctrl-a followed by 'x'*.