loader: Stop closing the network device when netbooting for loaders using the common dev_net.c code
FreeBSD change: https://svnweb.freebsd.org/base?view=revision&revision=r307238
The NETIF_OPEN_CLOSE_ONCE flag was added in r201932 to prevent that behavior 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