Stop closing the network device when netbooting for loaders using the common
dev_net.c code.

The NETIF_OPEN_CLOSE_ONCE flag was added in r201932 to prevent that behaviour
on some architectures (sparc64 and powerpc64) the default was left to always
open and close the device for each open and close of a file by the loader
because it was necessary for u-boot on arm.

Since it has been added, the flag was turned on for every arches including the
u-boot loader for arm.

This also fixes netbooting on RPi3 (tested by gonzo@)

For the loader.efi it greatly speeds up netbooting

Reviewed by: emaste, gonzo, tsoome
Approved by: gonzo
MFC after: 1 month
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D8230