DO NOT MERGE: Implement boot signature verification

Adds signature verification support to BootSignature.java.

Bug: 15984840
Bug: 18120110
Change-Id: Ib1a67573bdcd0f41e2d920f518de15993c8750e0
(cherry picked from commit 7999c089cfa82d63d4a28606c78b381b74509175)
2 files changed