uboot-phytec

pxe: support "devicetree" tag

The specification for extlinux.conf[1] states that "fdt" is an alias for
"devicetree". To date, U-Boot only implements "fdt". Rectify that.

[1] http://freedesktop.org/wiki/Specifications/BootLoaderSpec/

Signed-off-by: Stephen Warren <swarren@nvidia.com>