PXE: FDT: Add support for fdt in PXE

Now DT support is becoming common for all new SoC's. Hence it is better
to have option for getting specific FDT from the remote server.

This patch adds support for new label i.e. 'fdt'. This will allow to
retrieve 'fdt blob' from the remote server. This patch take care for
the following scenarios.

The usage of fdt is optional.
The 'fdt blob' can be retrieved from tftp or can be available locally
or can be absent.

If 'fdt_addr_r' environment variable is set and 'fdt' label is defined
retrieve 'fdt blob' from tftp. 'fdt_addr_r' is then passed along bootm
command.

If 'fdt_addr' is set and 'fdt blob' is not retrieved from the tftp pass
'fdt_addr' to bootm command. In this case 'fdt blob' will be available
at 'fdt_addr'.

If 'fdt_addr' is not set and 'fdt blob' is not retrieve from tftp pass
NULL to boot command. In this case 'fdt blob' is not required and absent.

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Acked-by: Jason Hobbs <jason.hobbs@calxeda.com>