This commit introduces a new clusters to the configuration to add
variables with `:append`.

For better readability, this change changes the name of the clusters
that were previously adding variables with `+=`. Now, those clusters
have been renamed replacind the string `APPEND` with `ADD`.

This change involves renaming of the former clusters:
- CONFIG_FEATURE-APPEND to CONFIG_FEATURE-ADD
- CONFIG_YS-APPEND to CONFIG_YS-ADD

A new cluster has been added to be associated with the `:append`:
- CONFIG_FEATURE-APPEND

The actions related to the above-mentioned clusters have been renamed
too.

---

The function `local_conf_merge_variables` has been modified to handle
differently the duplicates:

Case 1: all operators are the same
- multiple `=` -> merge with `=` raising a warning
- multiple `+=` -> merge with `+=`
- multiple `:append` -> merge with `:append`

Case 2: different operators
- with one `=` and multiple `+=` --> merge with `=`
- otherwise raise an error and stop the execution

Signed-off-by: Nicola Sparnacci <nicola.sparnacci@seco.com>