The function `local_conf_sanity_check` has been updated to consider also
the variables assigned with `:append` and `+=` in the `local.conf`.

The variables are checked over `*-feature.conf` files which are included
in the layers needing mandatory features.

The function shortly does the following:
- check if the variables in the `*-feature.conf` are in the `local.conf`

For each variable both in `local.conf` and `*-feature.conf`:
- if a variable is assigned in `local.conf` with a different assignment
  (i.e. `0` vs `+=`, `:append` vs `+=`, etc), return a configuration
  error.

- if a variable is assigned in `local.conf` with a different value
  than the one in the `*-feature.conf`, return a configuration error.

- if a variable is assigned in `local.conf`` & `*-feature.conf` with the
  same values and operators, the one in the `local.conf` are removed
  and the one in `*-feature.conf` is preserved.

- if a variable is assigned with `:append` or `+=` in the
  `local.conf`, the common values are removed from `local.conf` as they
  are already in `*-feature.conf`. For example:
    ```
    local.conf: VAR:append = " A B"
    *-feature.conf: VAR:append = " A"
    will result in:
    local.conf: VAR:append = " B"
    *-feature.conf: VAR:append = " A"
    ```
    the same applies for `+=` but it doesn't for `=`.

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