mach-snapdragon: generate fdtfile automatically

mach-snapdragon: generate fdtfile automatically

With just a few basic rules, we can generate the $fdtfile environment
variable to match the format used in Linux. This uses the root
compatible property inside u-boot, with specific handling for the
Dragonboard845c which is a special case, and for the qrb robotics
boards.

This is known to work on supported platforms, and lets us avoid having a
big lookup table.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Sumit Garg <sumit.garg@linaro.org> #qcs404
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>