nanddump: document, warn about future default --omitoob

nanddump: document, warn about future default --omitoob

To work as a proper inverse to nandwrite, nanddump must not dump OOB data by
default. This patch prints a warning regarding the future change. We also
suggest using the new flag, `--oob', for transitioning to the next release
with new default behavior.

Note that we are changing `-o' to mean `--oob' in the next release,
similar to `nandwrite -o'.  This is a break from previous behavior.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>