build_project_externals: always do a full externals build

Change the default for externals builds from incremental to full build as not all externals are compatible with incremental builds. Note that if there are no changes to the externals version, the already existing build will be fully re-used as before. This also removes the need for cleaning up after failed builds as the next build will start from scratch in any case.

One can use the new -i option to force an incremental build if needed.

Relates to ATLINFR-4752.

cc @akraszna