Revert "Revert "mkbootimg: Build mkbootimg with Soong.""

This reverts commit 55a78bf5.

There's no change in this CL from the initial landing. The earlier crash
was caused by manifest setup in build-tools and clang-tools branches,
which has been addressed by the following CLs.
https://android-review.googlesource.com/c/platform/manifest/+/769045
https://android-review.googlesource.com/c/platform/manifest/+/769044

Bug: 116698229
Change-Id: I33701c9f77e814257cc5c65ad7aa587835ae00ad
Test: TreeHugger