docs: Update Linux Flavour docs to describe adding debug-tweaks

Linux Flavour docs mentioned logging in as root with no password, but
this is only possible with debug-tweaks enabled, which was not
previously documented.

Closes #707



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

docs/build-flavours/linux-flavour.rst

@@ -30,11 +30,26 @@ Supported machines (default in **bold**):
 
 Build steps example:
 
+* Set up the enviornment:
+
 .. code-block:: console
 
     $ TEMPLATECONF=../oniro/flavours/linux . ./oe-core/oe-init-build-env build-oniro-linux
+
+* Enable ``debug-tweaks`` in order to create a default user by appending
+  the following line to your ``conf/local.conf``:
+
+.. code-block:: console
+
+    EXTRA_IMAGE_FEATURES += "debug-tweaks"
+
+* Start the build:
+
+.. code-block:: console
+
     $ bitbake oniro-image-base
 
+
 You can test the image built for the qemux86-64 target by issuing:
 
 .. code-block:: console
@@ -47,7 +62,7 @@ After successful bootup, you will be dropped into a login shell:
 
     qemux86-64 login:
     
-Default login is *root* without a password.
+If ``debug-tweaks`` is enabled, default login is *root* without a password.
 
 After login you will see the shell prompt: