Setting the nfsroot variable as well as the kernel bootargs for the
interface used to reach the server is specific to nfs, which are used
only if there's no initramfs to load from tftp. Move these statements
within the condition.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>