- turns out everybody was wrong, including me
- some (older?) ATF sources won't work, ever; thus
- introduce ATF_SKIP_LDFLAGS=yes to skip it completely
- introduce ATF_SKIP_LDFLAGS_WL=yes to only skip the `-Wl,` prefix
- this is for ATF's that pass flag directly to linker, not gcc
- artifact-uboot: hash atf-building code into artifact version