Use cboot by default on TX2 mender builds

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

See discussion at OE4T/meta-mender-community#8 (comment)

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

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

@@ -48,3 +48,6 @@ MENDER_ROOTFS_PART_B_NUMBER_jetson-nano-2gb-devkit = "2"
 # Overrides until meta-mender-tegra is updated
 BOOTENV_SIZE_jetson-nano-devkit = "0x8000"
 BOOTENV_SIZE_jetson-nano-2gb-devkit = "0x8000"
+
+# Use cboot by default on TX2 to avoid uboot slot alignment issues
+PREFERRED_PROVIDER_virtual/bootloader_tegra186 = "cboot-t18x"