Some variables may be set from DHCP requests, as shown below:

    barebox@ZII VF610 Development Board, Rev C:/ echo $global.hostname
    dev-rev-c
    barebox@ZII VF610 Development Board, Rev C:/ echo $global.net.server
    0.0.0.0
    barebox@ZII VF610 Development Board, Rev C:/ dhcp
    eth0: 100Mbps full duplex link detected
    T T T eth0: DHCP client bound to address 192.168.203.31
    barebox@ZII VF610 Development Board, Rev C:/ echo $global.hostname
    zii-dev-rev-c-1
    barebox@ZII VF610 Development Board, Rev C:/ echo $global.net.server
    192.168.203.1
    barebox@ZII VF610 Development Board, Rev C:/

These variables are used by the boot/net script, so it is necessary
to trigger DHCP before we set the TFTP file paths, otherwise booting
from net will fail like below and will need to be triggered twice:

    running /env/bin/init...

    Hit m for menu or any key to stop autoboot:    1
    booting 'net'
    eth0: 100Mbps full duplex link detected
    T T T eth0: DHCP client bound to address 192.168.203.31
    WARNING: eth1: No MAC address set. Using random address 8a:ee:18:f5:98:e7
    T T T T T T T T T T T T T T T T T T T T could not open /mnt/tftp/none-linux-dev-rev-c: No such file or directory
    ERROR: Booting 'net' failed: No such file or directory
    boot: No such file or directory

    type exit to get to the menu
    barebox@ZII VF610 Development Board, Rev C:/ boot
    booting 'net'

    Loading ARM Linux zImage '/mnt/tftp/none-linux-zii-dev-rev-c-1'
    Loaded initrd unknown '/mnt/tftp/none-initramfs-zii-dev-rev-c-1'
    initrd is at 0x82509000-0x835593ff
    Loading devicetree from '/mnt/tftp/none-oftree-zii-dev-rev-c-1'
    commandline:  console=ttyLP0,115200n8 ip=dhcp
    Ucompressing Linux... done, booting the kernel.

To fix this, bring the interfaces up before setting up tftpboot.

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