ARM: tegra: implement bootcmd_pxe

This retrieves a PXE config file over the network, and executes it. This
allows an extlinux config file  to be retrieved over the network and
executed, whereas the existing bootcmd_dhcp retrieves a U-Boot script.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
ARM: tegra: clear $usb_need_init each boot

$usb_need_init prevents "usb start" from being run multiple times for
each boot attempt, i.e. once for USB storage, another for PXE, and
another for DHCP. However, the flag that's used to determine when to run
"usb start" is never cleared, so a subsequent "boot" command will never
probe for a freshly plugged in USB device. Fix this so that new USB
devices will be probed once per boot attempt.

Signed-off-by: Stephen Warren <swarren@nvidia.com>