tegrademo-mender: Use cboot by default on TX2

Since uboot slot alignment issues are tricky, unless you know what
you are doing you probably want cboot as your bootloader for TX2/mender

See discussion at https://github.com/OE4T/meta-mender-community/pull/8#issuecomment-745564642



Signed-off-by: Dan Walkes <danwalkes@trellis-logic.com>