defconfig: Add android-verity support for system

Add android-verity support so that the system image is verified by
the kernel during the boot time. Also set the x509 key file to be
used for verification.

Change-Id: I17ffc2944ef9af556d9fdc05d62734de384dc27c
Signed-off-by: Binoy Jayan <bjayan@codeaurora.org>