dtc: give advance warning that "-S" is going away.

The "-S" option allowed the specification of a minimum size for
the blob, however the main reason for caring about the size is
so there is enough padding to add a chosen node by u-boot or
whoever.  In which case, folks don't really care about the absolute
size, but rather the size of the padding added for this -- which
is what the "-p" option does.  Having the "-S" just confuses people.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>