Fix leading space in DEVICE setting

live-boot will set the `DEVICE` parameter in `/conf/param.conf`. This
parameter specifies the default network interface to use, like eth0. So
only one Ethernet device is allowed here, but live-boot generates a list
with a leading space. For example it will set `DEVICE=' eth0'`. This
causes initramfs script relying on `DEVICE` fail.

Therefore just set the `DEVICE` parameter to the last connected
interface found.

Signed-off-by: Benjamin Drung <benjamin.drung@profitbricks.com>