tegra: use generic fs commands in BOOTCOMMAND

Modify tegra-common-post.h's BOOTCOMMAND definition to use the generic
filesystem command load rather than separate fatload and ext2load.
This removes the need to iterate over supported filesystem types in the
boot command.

This requires editing all board config headers to enable the new
commands. The now-unused commands are left enabled to assue backwards
compatibility with any user scripts. Boards (all from Avionic Design)
which define custom BOOTCOMMAND values are not affected.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
tegra generic fs cmds fixup
Signed-off-by: Tom Warren <twarren@nvidia.com>