linux-handhelds-2.6: Record teh fact that flash booting should be done with root...

linux-handhelds-2.6: Record teh fact that flash booting should be done with root=mtdN
* Per discussion on the list, reminding that root=/dev/mtdblockN form is deprecated
(requires mtdblock builtin for the sole purpose of looking up mtd device thru
block layer).
* Add that to generated sample HaRET config - after all, that's the bootloader
with most device coverage, so users of other bootloaders are expected to be
familiar with it still.