manifest: Track android15-release for vboot_reference

* AOSP effectively half updated this repo for 15 QPR1, then butchered it.
* It is also flagged to die soon, so jist track thr working revision while devices that use it still work.
* Use SHA1 as Google likes forcepushing tags.

Change-Id: I76a896b038562474b9015840ee3c4e46f2f8b7bb