docs: Document the change in the default password

Document the change in the default user/password for the production
images.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>

docs/build-flavours/linux-flavour.rst

@@ -47,7 +47,7 @@ After successful bootup, you will be dropped into a login shell:
 
     qemux86-64 login:
     
-Default login is *root* without a password.
+The default login name is **oniro** with **oniro** as a password.
 
 After login you will see the shell prompt:
 

docs/oniro-quick-build.rst

@@ -112,3 +112,5 @@ Once the image is built, you can run a Qemu X86-64 instance using the provided s
 
 If the host has a VT-capable CPU, you can pass the ``kvm`` argument for better
 performance. Check ``runqemu``'s help message for all available arguments.
+
+The default user name is **oniro** with **oniro** as a password.

flavours/linux/conf-notes.txt

@@ -28,3 +28,4 @@ You can also run generated qemu images with a command:
 
     $ runqemu qemux86-64 qemuparams="-nographic" oniro-image-base wic ovmf
 
+Then log in as 'oniro' user with 'oniro' as a password.