data_smart: Account for changes in append/prepend/remove in the config hash

bitbake wasn't reparsing when _remove items were added to its configuration
and equally, appends/prepends were also being badly tracked. This
change enrures these variables are accounted for in the configuration
hash.

[YOCTO #5172]

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