tegrademo-mender: Use cboot by default for TX2

When using TX2, use cboot as default bootloader, since this avoids the
additional complexity of u-boot partition mismatch and mender commit.

See discussion at [1].

OE4T/meta-mender-community#8 (comment)

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

layers/meta-tegrademo/conf/distro/tegrademo-mender.conf

@@ -36,3 +36,5 @@ MENDER_STORAGE_TOTAL_SIZE_MB ?= "${@tegra_mender_calc_total_size(d) - int(d.getV
 SYSTEMD_DEFAULT_TARGET = "finished-booting.target"
 
 IMAGE_INSTALL_append_tegra = " tegra-eeprom-tool i2c-tools tegra-bup-payload"
+
+PREFERRED_PROVIDER_virtual/bootloader_tegra186 = "cboot-t18x"