UBUNTU: [Packaging] build-avb: add rules to build AVB (Android Verified Boot) image

BugLink: https://bugs.launchpad.net/bugs/1927006



This patch adds support for generating Android boot.img using the
Image-dtb file & initrd.img prepared using ubuntu-core-initramfs tools
package.

Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>