rootfs-postcommands.bbclass: add a note to the login banner when root-with-empty...

rootfs-postcommands.bbclass: add a note to the login banner when root-with-empty-password logins are enabled

When enabled, users will see:

===
Poky (Yocto Project Reference Distro) 5.2.99+snapshot-1dac0d15f2d979128cb449b59bf5fd69b4ba5084 qemux86-64 ttyS0

Type 'root' to login with superuser privileges (no password will be asked).

qemux86-64 login:
===

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>