tegra: flesh out bootcmd

tegra: flesh out bootcmd

This implements a useful bootcmd for Tegra. The boot order is:

* If USB enabled, USB storage
* Internal MMC (SD card or eMMC)
* If networking is enabled, BOOTP/TFTP

When booting from USB or MMC, the boot script is assumed to be in
partition 1 (although this may be overridden via the rootpart variable),
both ext2 and FAT filesystems are supported, the boot script may exist
in either / or /boot, and the boot script may be named boot.scr.uimg or
boot.scr.

When booting over the network, it is assumed that boot.scr.uimg exists
on the TFTP server. There is less flexibility here since those setting
up network booting are expected to need less hand-holding.

In all cases, it is expected that the initial file loaded is a U-Boot
image containing a script that will load the kernel, load any required
initrd, load any required DTB, and finally bootm the kernel.

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