BTT devices
Those devices are based on IMX’s IMX287 SoC. The description regarding the btt family of boards (i.e. btt3 and bttc) is identical as the one for the already supported xea board.
Building
Make sure that CROSS_COMPILE is set appropriately:
$ make imx28_btt3_defconfig
$ make -j4 u-boot.sb u-boot.img
Now you should see u-boot.sb and u-boot.img files in the build directory.
For initial bringup - one can use uuu utulity to boot till u-boot prompt (USB connection with the board is required).
Flashing
Via U-Boot:
=> run update_spl
=> run update_uboot