Minor fixes to NAND code and docs

Minor fixes to NAND code and docs

Erase logic:
 - command invocation
    + treat "nand erase N" (no offset/length) as "erase whole chip N"
    + catch a few more bogus parameter cases, like length == 0 (sigh)
 - nand_erase() should be static
 - on error
    + say which block failed, and if it was a bad block
    + don't give up after the first error; try to erase the rest
 - on success, say which nand device was erased (name isn't unique)

Device list ("nand list"):
 - say how many blocks there are
 - split summary into two lines
 - give example in the docs

Doc tweaks:
 - Use @option{...} for DaVinci's supported hardware ECC options

For the record, I've observed that _sometimes_ erasing bad blocks causes
failure reports, and that manufacturer bad block markers aren't always
erasable (even when erasing their blocks doesn't trigger an error report).

git-svn-id: svn://svn.berlios.de/openocd/trunk@2724 b42882b7-edfa-0310-969c-e2dbd0fdcd60