utils: Implementing utility functions for boot_manager

This CL implements virtual class boot_manager and the utility functions
that will be used in boot_manager.

Tested:
This CL won't change any behavior.
Can only confirm that unit test passed.

Google-Bug-Id: 298918774
Change-Id: Ib26efa04990ed0f16316741bd4172a660968b3a9
Signed-off-by: Michael Shen <gpgpgp@google.com>
7 files changed