Add support for Virtual A/B

For Virtual A/B OTA feature, change to
  * add intial support for feature.
  * parse misc partition for virtual a/b merge status.
  * publish snapshot merge status in fastboot variables.

Change-Id: I454972cc2be673054c50e5de6de5b3abafbdf2fd
Signed-off-by: Rahul Shahare <rshaha@codeaurora.org>
6 files changed