rootfs-postcommands.bbclass: fix echo + '\n' in 'no password' banner

rootfs-postcommands.bbclass: fix echo + '\n' in 'no password' banner

The '\n' means hostname instead of new line in /etc/issues.

bash and dash have different behavior on echo + '\n'.
So we avoid this '\n' and use an extra echo "" instead.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>