cc/cc.go: Support per-image-variation "required"

Support "core" and "recovery" variants for now. Might add more image
types if needed.

Bug: 191369319
Test: Presubmit
Test: Inspect out/soong/Android-*.mk
Change-Id: Iebab29ed5d6d8fe9c66b6d6e56e00246d10c36b3