syslinux: Add support for DEVICETREE from bootloader spec

syslinux: Add support for DEVICETREE from bootloader spec

The bootloader spec says:

> `devicetree` refers to the binary device tree to use when executing the
> kernel. This also shall be a path relative to the `$BOOT` directory. This
> key is optional. Example:
> `6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.armv7hl/tegra20-paz00.dtb`

This is necessary for booting my NVidia Tegra TK1 device.  It uses u-boot
with syslinux compatibility.  In the syslinux files that come with the
device this is called `FDT`, but u-boot treats `FDT and `DEVICETREE` as
synonyms.

See also: [f43c401 in u-boot].

[f43c401 in u-boot]: http://git.denx.de/?p=u-boot.git;a=commit;h=f43c401b72bb0db43ab0b55c4a79e1f4889d3aa2

Closes: #1411
Approved by: cgwalters