[CORE] Add variable merging & FEATURE-APPEND cluster

• Add cluster FEATURE-APPEND to add to local.conf a feature variable with += assignment.

• Add variable merging into local.conf. For example:

  VAR = "test"
  VAR += "second-test

  ends up with:

  VAR += " \
     test \
     second-test \
  "

Note:

1. In the case of a variable with multiple = assignments, the values are merged but a warning is raised.
2. In the case of a variable with multiple = assignments, the values are merged and the operator = is used.
3. In the case of a variable with multiple mixed assignments (+=, =, :append) the += operator is used.
4. In the case of a variable with multiple mixed assignments, a :append is treated as +=.

Version bump type: [MINOR]