Revert "Use stamp file rather than grouped targets for TF-A."

This reverts commit 17a8b9700b06e885e5eba17215c7e2103e61b401.

Reason for revert: With https://android-kvm-review.googlesource.com/c/prebuilts/+/2640 and related changes we now use a more recent version of make and can use grouped targets.

Change-Id: I23c839a8590de3e7ce536721cf5561dfb82b1cef