Using CONFIG_OF_LIBFDT

I've added the following Important Safety Tips to the u-boot-fdt status page
<http://www.denx.de/wiki/UBoot/UBootFdtInfo>
based on feedback from Jon Diekema (thanks, Jon):

* Need to use the latest device tree compiler (dtc) to generate a Version 17 blob
* libfdt needs V17 in order to write to the blob

* Need to specify a size with fdt addr
* By default, the blob has the minimum necessary size for its
compiled content. This means there is no room to add a "chosen" (or other) node.

* Using CONFIG_OF_LIBFDT modifies bootm so that it does not auto-generate nodes ("chosen", "u-boot-env", and "bd_t") when it runs. With CONFIG_OF_LIBFDT, the boot script or the user must use the sequence:
* fdt addr command to set the blob address
* fdt chosen to generate/augment the chosen node
* fdt env to generate the u-boot-env node (optional)
* fdt bd_t to generate the bd_t node (optional)

Bear with me, we'll get this smoothed out...
gvb