qemuboot/runqemu: fully form the ip= kernel parameter

New systemd is actually parsing this in systemd-network-generator
and fails if it is not fully formed. 'off' means 'static ip, do nothing':
https://www.kernel.org/doc/Documentation/filesystems/nfs/nfsroot.txt



Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>