Remove un-implemented and dubious "nand copy" command.

Doing this efficiently would mean doing the copying on
the target CPU, instead of back and forth through JTAG.
If anyone ever needs this functionality, that's what
they should implement.

Also, update on-line "help" for "nand dump" to display
its two optional flags; and for "nand write" to display
a recently added flag.



git-svn-id: svn://svn.berlios.de/openocd/trunk@1883 b42882b7-edfa-0310-969c-e2dbd0fdcd60