fdt: Set kernaddr if fdt indicates a kernel is present

If kernel-offset is specified in the fdt, set an environment variable
so that scripts can access the attached kernel.

This can be used by a packaging program to tell U-Boot about a kernel
that has been downloaded alongside U-Boot. The value in the fdt is
the offset of the kernel from the start of the U-Boot image, so we can
find it just by adding CONFIG_SYS_TEXT_BASE.

It is then fairly easy to put something like this in the environment
variables in the board header file:

		"if test ${kernaddr} != \"\"; then "\
			"echo \"Using bundled kernel\"; "\
			"bootm ${kernaddr};" \
		"fi; "\
		/* rest of boot sequence follows here */

Signed-off-by: Simon Glass <sjg@chromium.org>