libsnapshot: Track the source slot across reboots.
The purpose of this is twofold. First, we need a way to detect whether
the device has booted into the new slot after an update. Second, we need
a quick-and-dirty test for first-stage init so it can avoid searching
for snapshots if it doesn't need to.
We can solve both of these problems by storing the source slot in
/metadata/ota/snapshot-boot. The existence of the file implies a more
complex check is needed, and the file stores the source slot.
Test: libsnapshot_test gtest
Bug: 139204329
Bug: 139089801
Change-Id: I919dd1c27a65734c61f3a9bdffefb32bdebd723b
4 files changed