Run tftpboot not tftp for network downloads

"tftp" only works because it's an abbreviation of "tftpboot", and is
unique within the typical set of enabled commands on Tegra devices. Fix
the script to use the full command name "tftpboot" so that it works
even when other TFTP-related commands (such as tftpput) are enabled.

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