data_smart: Add _remove operator

There are long standing complaints about the fact its very difficult
to remove a portion of a variable. The immediate request is for a -=
and =- operator. The trouble is that += and =+ are "immediate"
operators and are applied straight away. Most people would expect
-= and =- to be deferred to have the effect most people desire and
therefore implementing -= and =- would just make the situation more
confusing.

This deferred operation is much more similar to the override syntax
which happens at data store finalisation. The _remove operator is
therefore in keeping with the _append and _prepend operations.

This code is loosly based on a patch from Peter Seebach although it
has been rewritten to be simpler, more efficient and avoid some
potential bugs.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>