common/image.c: align usage of fdt_high with initrd_high

The commit message of a28afca5 (Add uboot "fdt_high" enviroment variable)
states that fdt_high behaves similarly to the existing initrd_high.
But fdt_high actually has an outstanding difference from initrd_high.
The former specifies the start address, while the later specifies the
end address.

As fdt_high and initrd_high will likely be used together, it'd be nice
to have them behave same.  The patch changes the behavior of fdt_high
to have it aligned with initrd_high.

The document of fdt_high in README is updated with an example to
demonstrate the usage of this environment variable.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Simon Glass <sjg@chromium.org>