Likewise the corresponding support for syslinux introduced by commit
c5112c25e4, this one enables writing devicetree
filename into the uEnv.txt environment file for u-boot.
Since u-boot does not strictly defines variable names, here 'fdt_file' was
chosen as it appear to be one the most frequently adopted names in u-boot
default environments. Outer boot logic should of course comply with this choice
and use $fdt_file as the device tree file name to pass to boot commands.
This was tested on a custom board booting with u-boot.
Closes: #1590
Approved by: cgwalters
