nvida-tegra-armv7: disable fastboot

We did set these variables, so it is possible to flash boot and root partitions separately for booting with stock partition table. This is useful for U-Boot as secondary bootloader. But it's not good for U-Boot as primary bootloader because the resulting images have boot and root partitions separated, so users have to create partitions themselves and this is very confusing.

I'm suggesting to disable it in favor of doing U-Boot as primary bootloader, this may be harder for users, but it needs to be done only once, and this way is more fancy and is supported on more devices than secondary bootloader.

If someone still wants to use this port with U-Boot as secondary bootloader they can do "pmbootstrap install --split" and "pmbootstrap export" to get 2 separate partitions to flash.